Holy crap! <Late Main Quest Spoilers>

[Obsy]

Since they do possess bodies to exist in the mortal world (loosely remember what Minfilia said), I wouldn't be surprised if Lahabrea would jump back and forth between bodies depending on when he was needed. Whichever host was closer depending on what what he wanted do.

Hm, I'm not sure. Given the shown Ascian ability to teleport in and out at will regardless of their host body, I can't get on board with it being due simply to "convenience".

My thoughts are that Lahabrea possessed Thancred for two reasons:

1) to glean information related to the Scions of the Seventh Dawn (i.e. the location of The Waking Sands, from which he then ported Livia & Co. in for Stabby Time™), and

2) to twist the knife during the big reveal, as evidenced by Minfilia's outcry (and probably a few of our own). From a storytelling perspective, it's obvious the writers wanted to really drive home the notion that nothing was safe; our own companions could be ripped from our hands, corrupted, and used against us.