It is b%&##+$% to say that there are necessary roles that must be filled. It is not b+#$&&*%, however, to a) point out that there is already someone playing a paladin, for example, which would mean your character will be very similar to another, and b) let the party lacking some important capability to find it a serious problem. It is not the GM's job to only put in monsters that a wizard only party can easily deal with, nor to provide multiple copies of loot for gunslingers/whatever.
Warning people that their character idea might not be the most fun in the current group (like a Sorcerer joining a group with one of those Wizards who have every spell there is), or warning the group that they lack a vital skill (like noting "You guys lack a healer."), is the GM's job. Forcing people to play anything isn't. If the Sorcerer wants to join the group with the Wizard, or the group wants to go healer-less, that's fine...but I feel a warning is in order.
Additionally, sometimes a suggestion of a minor modification or for a specific version can be good. For example, while I wasn't the GM in this case, I recommended an Urban Ranger to one of my fellow players in an upcoming RotRL game since he was planning on making a Butterfly Sting TWF build, hadn't decided on a class yet, and we needed somebody to deal with traps. I also suggested the Healing Patron to the guy going Witch since we lacked any other real healer. I would've likely made similar recommendations (and that's all they are, recommendations) had I been the GM, and I think that's acceptable.