Tenkar's Tavern ask Is The Thief a Needed Member. He asked if the thief class is truly needed or if an adventuring party could get by without one. How important is a thief to a group? Essential or just nice to have? This got me thinking latterly...
Quite a few classes do not become needed until a player starts to play one. It is a backwards need. The thief is an excellent example of this. If no one in the party is playing a thief then a DM tends to not add traps, or run adventures that require pickpocketing or include lots of locked doors or anything that a thief excels at. Sure a DM might still add a locked door or trap but with the intent that there is nothing the party can do about them other than soaking the damage or bashing in a lock. Such DMs would not write an adventure where the adventure would stall because there is no thief in the party.
However, once a thief is being played, then suddenly there are all sorts of traps, locks and other things in adventures. Adventures will stall if that thief can't pick the lock. DMs make sure to add in things for a thief to do. It follows the mantra of "let every player shine at least once". Thus thieves and thiefly skills are not needed until a player starts playing a thief.
This expands out to other classes as well...
Once a bard joins a party, there are suddenly more "social" encounters.
Once a druid or ranger joins a party, there are suddenly more nature/animal encounters.
Once a paladin joins a party, there are suddenly more encounters with nobility.
It could even be said that the cleric/healing class follows this as well. On one hand, combat is a driving force of rpg games and thus a cleric is essential to the survival of a party. However, if for some reason there is no cleric in a party, damage scales would go down...or potions of healing would begin to be found as treasure a lot more often.
DMs react to what the players have chosen as their classes. No class is required...until a player starts playing them.