I just bought the base set and a couple of small expansions. So i was reading through the items when i saw the soul gem item. Is the item suppose to be so useless? Is the upkeep suppose to activate irregardless whether there is or isn't any monster trophy on it?

You choose in the upkeep phase whether to discard the soul gem and reap the benefit. If that isn't what you're asking, I didn't understand the question.

So you can choose not to use it during upkeep? I thought upkeep is compulsory?

Upkeep Phase is compulsory but using cards that can only be used during Upkeep is optional. Having Sanity 1 and forced to cast Voice of Ra just because it has Upkeep phase listed when you can use it, even if you can't succeed ('cos your Lore is 0 and your Focus 1), would be pretty daft. Or Arcane Insight with a character who has Lore 4 max and no Tomes.

So the answer is that we should interpret the cards reasonably. Ok thanks.

I think they should have writen it differently using upkeep and upkeep phase.

When an action is compulsory, it is usually preceded by "you must" in the text.

DarkFate said:

The isn't Magic: the Gathering. The upkeep phase is just another phase; a phase where things CAN happen. Nothing is REQUIRED to happen then unless the rules or the card explicitly say so. The game format of listing which phase a card MAY be used in is consistent across all phases, and if it's mandatory, the card will say so in black and white.

It sounds like you're inferring something from the word "upkeep" that is not true in this game. Stop transferring rules from other games onto Arkham. =P

The rules book explictly wrote " Each player must perform all all Upkeep actions listed on his investigator's cards every turn." I didn't made it up. I have played dozens of cards game and boardgames. I know full well not to mix up the rules from different games.

The case in this point is that soul gem and (for example) bank loan has the same upkeep format but 1 need not be activated and the other must.

I don't know why the book says "must." You're required to do upkeep-related things like rolling for a Blessing or Curse or Retainer, but in general items and skills that are activated during the upkeep are optional.

Stuff that says "exhaust to..." or "discard to..." should be optional.