As for your (as well as a decent amount of people here) steadfast assumption that on Kelly's return he'll be playing on the 4th line...what happens if indeed, on his return, the lines return to they way they were before pre-injury bug hit? Loui back on the 2nd, Smith back on the 3rd (as it appears he's cooling off some) and reunited with Kelly & Soderberg, and Merlot being Merlot, and Spooner back to Providence? The same lines that were first in the Division & Conference on December 8th (despite the assertion that the team was worse off then)?
It would be a questionable move at best, as that third line carried a negative plus minus on a team with a massive goal differential, Loui struggled to get anything going with Bergeron and Marchand, a lack of clicking that seemed to hinder the offensive production of that whole line, and Spooner would be in a much worse situation for learning the things he needs to learn, having Craig Cunningham to watch instead of Krejci, Bergeron, Kelly, and Campbell...not to mention the coaching difference between the two levels.
We go into playoffs (healthy assumption) with Caron as our 13th forward, and Spooner not playing an NHL game for months...which is obviously less depth than having the luxury of Thornton as a 13th.
The lineup is significantly better with Spooner in it than Thornton on paper. Obviously we haven't seen what it could potentially do because it's never been fully healthy with Spooner involved....but if last night was an indication, that third line could create magic.
Kelly was on pace for 17 points in a full season. Not something we can have in the top 9 with the way we spread minutes. I think it's a disservice to Kelly, as putting him between Soderberg and Loui forces him to play a game he simply isn't made for. On the 4th he has linemates that compliment his strengths, making the B's a better overall team.
Spooner's development gives the B's a chance to have some seriously beautiful depth, a luxury in the regular season, a necessity in the playoffs.