A few things:
1) Marcus Thornton is a solid basketball player, albeit overpaid. He's a 45/35/85 shooter on a high volume of threes, which makes him a pretty damn efficient scorer. He can also handle the rock a bit. No defense to speak of but we have that already with Smart/Bradley/Rondo. Could easily see flipping him at the deadline and scooping up another pick or young asset, a la Jordan Crawford.
2) Tyler Zeller improved his game dramatically last year, even though his MPG declined. On a per-36 basis, his PPG, RPG, BPG, and FG% all improved materially, as did his PER (11.0 to 15.4). He's 24 and cost-controlled through 2017 for ~$2M/year. Even if all he becomes is a backup center, getting a solid, healthy, young center for $2M/year is a coup.
3) That 1st rounder is potentially really valuable. If Lebron goes back to Miami (and Woj says most insiders still think that is the likely scenario), Cleveland will be stuck with a mediocre team in the East. They had the 10th-worst record last year and I doubt they improve a whole lot from there over the next two years. They will be like the Hawks - decent but not great. That means we have a good shot at a late lotto or late teen's pick waiting for us in 2016.
So in summary, we traded a 2nd and used a rapidly depreciating asset to pick up a player the team could use to get better / could be flipped for more assets, a nice, cost controlled big, and another 1st. Nothing wrong with this deal in the least.