Yes. Indeed. I will post my opinions. They are highly regarded. Or so I've heard.
I've never tried playing with annexation off, since I'm an imperialistic chauvinist.
However, I will offer my highly theoretical musings on the issue!
Annexation off will slow down the expansion of many (not all) nations. Most notably, Russia, Poland, France and Austria. Still, you can expand pretty swiftly anyway, you just can't take the capitals of the unfortunate wimps.
I imagine annexation off would be better for multiplayer, though. As you said, there would be less moshing and jockeying for positions in central europe that way...
Still, I like having the option of annexing (screw historical correctness, if I wanted everything to go according to history, I would be READING about it, not playing EU...)
As far as singleplayer games go, I doubt I will ever turn off the annexation option (possibly on a dare) because it leaves me less freedom to go about my game the way I want to.
In multiplayer, I would like to try both kinds, and I doubt you will be able to be as agressive in mp as in sp, since there will be people there to actually take notice and respond according to your expansion...
Well, them's my thoughts on the subject anyway...