The single app controlling everything works much better. I have my iPhone syncing over Wifi to iTunes on my Mac Mini and I don't have to think about anything. It just syncs on its own anytime I plug it into any charger or anytime I go into Settings on my iPhone and click the sync button.
I can't imagine having to worry about multiple apps when all I want to get is get all my content on my device. And then managing space on your device would require juggling between multiple different apps. It's not hard to understand why its best to have one tool to manage all your media content.
I'd imagine they would have a sync solution that didn't involve opening a bunch of different apps
..and this one-app sync solution is called.. iSync!
Having iTunes doing the syncing is mostly just historical, and made sense back in 2001 since iSync wasn't around when the iPod came out. Having the iPhone sync with iTunes also made sense because, to the end user, it was basically an iPod with phone functionality, and iTunes had always been used to sync iPods.
But I think at some point, having a music jukebox software sync photos, contacts, calendars, apps, documents, books, games, etc will become like hammering a square wooden peg into a round hole: It'll sort of fit, but it sure won't look pretty.. Have you seen how many tabs there are on the device screen in iTunes?
I'm sure many other people will have wondered at one time or another why the iPhone didn't sync using iSync when all other phones did ('cept maybe the ROKR). Apple hopefully will also have wondered this, and will now use iSync for the purpose it was created, instead of letting it languish and having iTunes become this complicated media/device manager software...
As for having to open multiple apps to sync, iTunes already gets around this when it syncs contacts, photos, movies, etc. You just do it in iTunes; you don't have to open Address Book, iPhoto, iMovie, etc. It's not a stretch to see iSync doing the same thing