You also have to consider that a lot of people who theorycraft optimized builds may also play in Adventure League - and you can play from level 1 to level 20 in that. I've got one character who's build didn't really "come together" until lvl 12, but he's now well equipped for high level play. The hard part was surviving levels 5-10, but I did it.
Other times, the dip into another class is more about building on the character theme and strengths, and the abilities of a single class don't really fit. (A common one for me is taking 1 level of fighter so I can play a melee oriented arcane caster in heavy armor.)
Other times, the dip into another class is more about building on the character theme and strengths, and the abilities of a single class don't really fit. (A common one for me is taking 1 level of fighter so I can play a melee oriented arcane caster in heavy armor.)