Alright, let me try to be constructive here.
Folks arguing for the Spider, specifically Teveshszat, Huitzil37 and Drudenfusz. Please present your reasons for why the Spider's existence is beneficial for the setting, in a way that can't be achieved by:
a) The Scorpion Clan;
b) The Otomo and other existing Imperial Families;
c) AND the Shadowlands Horde.
1: It allows the card game to be made such that politics/courtliness is a thing everyone participates in as part of the game. As it stood, if you cared about military, you didn't really care about courtliness in most cases, and those victory conditions did not interact. The wonky or missing interaction between victory conditions was a bad thing about the CCG; having a faction that never participates in courtliness makes it far, far more likely that courtliness will be something decks can ignore, and then we're right back to the same problem. If you want a game where you cannot ignore courtliness, you must have built your setting and its factions in such a way that none of them can do so. Since the Scorpion Clan allegedly work for the good of the Empire, they cannot carry the weight of being the Big Huge Bad Guys, the Otomo are not antagonistic at all, and the Horde do not participate in courtliness.
2: Making a setting in which "summons oni" and "is invited to tea and kabuki" are not mutually exclusive makes the setting finally pick a side between "This is a story about good vs evil" and "This is a story about order vs chaos". The Spider are evil but may participate in the system because that is harmonious, and saying "This isn't right, these people are dangerous monsters, they don't get to follow the rules!" is disharmonious and damaging to Order. Knowing what your setting is about and pushing it is infinitely preferable to the old way of jumping from one foot to another whenever one of your inadequacies was pointed out. Samurai drama, in which people are relentlessly punished for doing the right thing, works much better when you have this giant thing to point to to say "Look, these guys are evil as all hell, but they follow the rules and that makes it fine". You could go around this, by emphasizing more evil aspects of the other Clans, but that's very harmful for different reasons. Better to have Clans who are mostly sympathetic but say "Hey man, Spider get to sit at the table because rules are rules" to establish that Rules does not map to Good, than to accomplish this by having Clans who are more evil in non-supernatural ways -- since non-supernatural evil other than murder drives players away a lot more than murder and supernatural evil. The Scorpion claim to be good guys and thus do not force the conflict between Lawful and Good on behalf of others (they go through it themselves), the Otomo are not evil or chaotic and really aren't relevant to any of these, and the Horde is necessarily outside the system and do not force the conflict between Lawful and Good.
3: Making a setting in which the Big Bad Guy is a participant in the systems of politics and culture draws attention and focus away from the big bad guy, not toward it. When it is 8 Clans and The Malicious Power Outside Who Will Destroy Them, we always feel as though they need to be doing more to address The Threat, and stories that do not have anything to do with The Threat seem cheapened. If we know The Threat is within the system and operating by rules, we can believe in-character assessments from people in the setting saying "We know these guys are a threat, but stopping them is just one of many interests we have to balance our resources between." Characters focusing on other conflicts are not doing so out of ignorance. The Spider and Jigoku may want to destroy everything, but it is clearly a longer-term goal that does not require everyone else's immediate attention. This goal can obviously not be accomplished with Otomo or Horde, but could in theory by leaving out Spider and Horde altogether; but then you're also destroying the Crab and removing horror stories altogether, which probably isn't good.
Huh. Okay.
I asked a question about the setting, your first point was about improving the card game's mechanics. I'm not even going to address that, as it is utterly irrelevant for the purposes of this thread.
As for point #2, I think you're drawing a fake line between good/evil and order/chaos. In this game's setting good = order, and evil = chaos. They are practically synonyms, and there has been plenty of text (from myself and better writers/more knowledgeable than I) about why the setting is so. So, what I'm seeing here, is that you're first trying to draw on a non-existing conflict to justify something. And second is that your point requires, has as an absolutely mandatory condition, the fact that "oni-summoner" isn't seen as something absolutely horrid and despicable by the rest of the population. And, unless you remove ALL the history of L5R (I'm not talking just the past 20 years, but the 1200 years of setting history), that is just impossible. You would need a rewrite from as far back as the Fall of the Kami for that argument to hold. And if you're rewriting everything, well, then it's no longer the L5R that 8/9ths (that's about 88.89%) of the player-base follows - touching here on an earlier point that stated that you can't expect or wish for the majority to change for the sake of the minority.
Regarding your point #3, it requires for the rest of the game (ie, Imperial Faction and other 8 Clans) to hold what me, Gunichi, Killjoy and plenty others affectionately call "the idiot ball":
The Spider and Jigoku may want to destroy everything, but it is clearly a longer-term goal that does not require everyone else's immediate attention
It's important to note that the "stopping them is just one of the many interests we have" is a fake argument. It has been shown, time and time again, that regardless of differences, the Clans come together when the purpose is to defeat the Shadowlands/Taint - other interests be damned. It's a matter of priorities, and this one is way, way at the top - simply because it's a matter of survival.
There is however one silver lining to this, and that's the Susumu. As the non-tainted, highly political faction of the Spider, they already do exactly what you're describing. But please, PLEASE note the keyword here: Non-Tainted. The Susumu are the human evil within the Spider - an evil that is also present in all the clans (Scorpion and Imperials chief among them), and they are interesting exactly because of that. They don't summon oni, they don't participate in maho, they study necromancy or invite zombies for dinner. They are human, and that is why they rock. That is also why they would be one of the few Spiders that could survive an extinction post-Kanpeki.
Ultimately, what I get from your arguments is that you would like to see the monsters and the humans all together socializing. I'm sorry, but that's something that would work in D&D, would work in Shadowrun, would even work in some LoTR stories. But, unless you're specifically going for a KYoD-type of setting (which you are NOT), then it is something that has absolutely no place in L5R as it stands.