Because of the unique way planescape is played in dnd, i decided on removing two skills: Gather information and diplomacy
There wasn't much problem with gather info, it's just few more ideas i need to give players information if they don't find it
But we still argue about diplomacy. I don't like the skill because the player can say something wrong or useless or just explain in three words what they want to accomplish and roll diplomacy
Their argument is that removing diplomacy weakens the charisma attributes (and they're very right) and it's unfair to untalkative people who want to play very convincing characters (which i find a stupid thing to do, but they do have a point)
Then again, i like other charisma skills like bluff, handle animal or intimidate because those skills are not about what you say but how you say it. But my politics is that the listeners either like you and what you said or they don't. My NPC-s don't roll diplomacy against their PC-s so i don't see why it should be the other way around 
The biggest problem is that they have a lot more experience DM-ing then I do, but not in belief-based planescape setting, so i come looking for advice from the experienced planescape DM-s.
How do you handle diplomacy skills?
Also, what about gather information? How helpful is that to a human visitor in a town full of fiends or modrons?
Ditto for modrons, you can work through it slowly if you know the right forms to fill out and requests to make. The way I figure it, gather info is a combination of knowing who to bribe, how to bribe them, with what... be that buying a lady drinks and chatting her up, or offering straight up jink. 
I don't run often but I have found diplomacy to be useful, not so much as a skill to determine success but as a buffer for player to character problems. The use of social skills isn't just a Planescape question after all.
I once had a girl in an alternity game, she wanted to play a linguist, xenobiologist diplomat sort of character. Knowing her and how extroverted she was, I went "Sure, sounds good - the group could use someone who's not going to shoot the aliens on sight." Cause I knew she could do it.
What I didn't account for was her *nerves*. New player, playing in a game with her boyfriend and wanting to impress him, she put a lot of pressure on herself to "do well" to the point where she'd mess up her interactions just because she was that scared of messing up!
Diplomacy saved our butts at one point, she had to do the negotiaions with the big bad godfather like alien on this one planet, and for the life of her couldn't do it, was honestly about to quit the game. I remembered she had diplomacy, calmed her down and had her roll. Where - thank goodness - she rolled a critical success. My words at the time, "Ok, your character knows how to do this - at this point, just take a breath, say what you're going to say, your character will cover for it if you mess up - I'll let you know if something's wrong, we'll rewind a little and go through it again. There is no way you can mess up at this point, ok?"
Calmed her down completely, and by the end of the conversation she was fully in character for the first time in the game and had managed to negotiate bonus pay. What I *didn't* tell her was that I didn't change a single bit how big and scaley reacted to her based on that roll.
Diplomacy rolls are a good way to reassure players who are panicked by the idea of needing to RP. It also gives you a chance to subtly teach them how to actually be diplomatic, by fixing things when they're wrong and rewinding to do over because 'your dice roll will save your butt here'. There are folks out there who really just don't know *how* to do those sort of scenes, but if shown how - they pick up on it really fast.