D&D is a 'team game' and it's suggested for parties of 4 or more players. But the easier the game is to play, and the more it supports smaller parties, the better the game is in the long-run. Getting 5 people together to start playing is orders of magnitude harder than getting two people together to start playing. But once you have a game running, addiing people to the existing game is pretty easy.
If you can start with two players, you can add players until you reach your 'happy point'. But if you can't start without the 'minimum number of players' you can't ever get started.
Class design should reflect that. The game should be interesting and playable with a single character of a single class. It should be BETTER with more players, but the game should be POSSIBLE with just one.
I really find that Fighters can be interesting as solo-characters for only a very short time before the game breaks. Rogues, Clerics, and Wizards can pretty much solo at all levels of play. Obviously the casters have spells that help address some 'deficiencies' that they have; the non-casters can use potions for some of the caster type issues; but rogues can do more of that... Once a door gets arcane locked, the Fighter literally gets stuck.
I think that having out-of-combat abilities is important to ALL characters. It's a requirement to enable solo-play, which is desireable, even if group play is MORE desireable.