Why can iPhones only be synced with one iTunes library?

So I'm out on the road in Ann Arbor, MI.

My iPhone has a bunch of music on it that I've put on from my music collection on my Mac mini at home. I now want to add a couple of new tunes which I've put on my MacBook.

Can't do it. Nope. No way. If you set your iPhone to Manually Manage Music and Videos in iTunes you get this:

No I don't want to erase this iPhone and sync with this library! I just want to add a tune or two! What's so hard about that? You can do this fine with other iPods, but not iPhones.

I know - I can use PodWorks to copy all the music off the iPhone onto the MacBook and then sync with the MacBook, but that just seems like such a pain in the rear. C'mon, Apple - how about it?