Any thoughts on the matter?

If you want to play a Rogue Trader who is a Calixian Privateer, a Swashbuckler and a Xenographer, I don't see why not. As long as the character makes sense narratively, I wouldn't be against it.

If they are appropriate for the character then why not?

No problem, if you can make them fit, both story wise and mechanically. If you are trying to twink through it, I might try to dissuade you at first, but more than likely, go for it.

For instance, if you wanted to create someone who was a fighter god, you might manage to take your Void-Master, and squeeze in Flight Marshall ACR. Over time, as his myth grows, and he begins to seem like a Wedge Antilles, he could also grab the Legend of the Expanse ACR. Now, you're a stunt-god who can inspire fear in even the most jaded Raider pilot. I had a character idea where I thought about a character who had both Xenograpgher and Cold Trade Broker, combining someone who is an expert with aliens, probably even in good with them, and able to legitimately acquire their stuff, while also well-connected, and able to sell that stuff for a tidy profit, without having said aliens come back, looking for their relics. CTB and Acquisitionist might go nicely, too (yeah Good Rep without being an EA).

Just make sure that they aren't trying to squeeze in cool, stacking things, just to stack them, without having to petition for an Elite Advance. If it doesn't fit in well with their story, and with your current one, say nope, and stick to it.

I certainly don't mind it, though I try to be a bit conservative with things. Unless the game drags on forever, three really ought to be enough, and they have to make sense.

I can't imagine my players asking for more than two, only one of them has even brought it up. He'll be playing an Ork Mek Boy but was wondering if there is anything he could do to diversify before rank 4. He'll be happy to hear I didn't miss any kind of ruling disallowing it.