Thinking about my initial reaction to the switch ("Why not just have the Joycons permanently attached to the system and include a Pro Controller?"), the answer seems to be two-fold:
1. They want portable single-system multiplayer to be a thing (two+ people each with one Joycon, plus the kickstand), which I understood from the reveal trailer but I'm doubting makes much sense in 99% of situations.
2. They want to bring back the Wii Remote and Nunchuk. This is what I didn't understand/see coming from the reveal trailer. Two Joycon can completely simulate the Wii Remote and Nunchuk experience (but better, since each one has not only rumble but "HD Rumble," plus four face buttons PLUS two should buttons PLUS an analog stick.)
Factoring in both, the Joycon setup they've gone with makes a lot of sense. Personally I'm not keen on playing Wii-style games again, but it does return the freedom to make such games to developers (who it must be said during the Wii era, did come up with a lot of inventive if not completely fleshed out experiences.) Like, I still prefer RE4 Wii Edition because of the IR aiming and would have skipped a Wii U HD version cause of that (assuming it didn't support Wii Remotes, which seems likely.)
By default the Wii U dumped the Wii Remote concept entirely and Nintendo's answer was a shrug and "Eh you can use your last-gen controllers too." Devs couldn't count on it at all. But on Switch every system comes with not one but two Wii Remote-like controllers. And a pro controller of sorts, too. That's quite a bit of creative freedom. Will be interesting to see what devs do in the middle-to-last half of the life of this thing.