I agree that overcoming shortcomings of a lopsided party distribution is sometimes fun, at least for me. But letting players play what they want, and adjusting encounters to fit the party composition seems more fun for some players than one of them having to play a defender even though nobody wants to.
If the players find it fun to make a balanced party, cover missing roles, find what works best with the other PC's, great, but if 2 people want to play wizards, 1 has his heart set on a fey warlock, and 1 player is saying he can play whatever fills a gap, the "whatever" player may decide to go with a paladin to protect these squishies. They end up with no leader, and as DM, I'd just make adjustments to maybe reduce monster damage, and use lots of minions, or hand them an item or two that gives them an in combat way to trigger a surge, or something that makes secondwind more potent like a Cloak of the Walking Wounded.
There is nothing wrong with the DM making adjustments to the game for good challenging encounters. Sometimes a group works like a well oiled machine, and the DM has to take some measures to challenge them at least occasionally. Toning down encounter strength for a less than organized group is no different.