But the psychic power argument is exactly why one should have a list on hand, to show and prove you have taken said power.
At the hobby store I work at, I have to arbitrate countless games because the kids don't write lists and end up arguing whether characters actually payed for their gear they use. The only way to prevent characters from "magically" gaining need gear is to have a list clearly present. And I've seen people with stacks of different lists so the only way to insure someone uses the same list through the whole game is to present it beforehand. If you play with friends and people you trust this isn't a problem, but with different players it become a problem.
Also the reason I believe people should always draw out their deployment zone is again that you are just playing a game. Certain armies benefit from being closer or further from their enemies and to insure fairness there are set rules to govern the game and enhance its playability. I'm sure we all remember are friends "invincible" G.I. Joe from our childhood.
Again if a house rule works for you and your friends then who am I to say you can't though. Just remember that it doesn't work for everybody.
As for my opinion if somebody asks if a unit has a piece of gear you should tell them, if not to help smooth out your playing experience then to strengthen trust between players. That doesn't mean you have to go out of your way to tell them what gear you have.
Many of my Space Marines have died needlessly because I didn't bother to checkout what I was assaulting.
"What do you mean it has seven attacks that allow no armor or invulnerable saves?!"
<- Me
In the end though remember, it's just a game.
