Thornton on the first line could be for all sorts of reasons, beyond thinking he'd be a good fit style-wise:
1) Since he was playing in Europe he's closer to being game ready, may as well take advantage of it.
2) A strong personality with the two young forwards they want him to impress upon.
3) If you want Thornton to be a presence in the locker room he has to have a significant role on the team. The young 'uns aren't likely to find the words of a 4th liner as compelling as (See point #2).
4) It's part of an effort to push the prospects and new players who will likely find their way into the bottom 6, if at all. Putting Joe in the top 3, for now, allows Keefe to get more looks on these guys.
5) I was going to put something funny here but it's not that funny.