you need to draft guys who can turn into NHLers. Not every skilled guy is going to make it.
TB traded the 20th pick in the draft to grab a 3rd line guy.
At this point in the draft you want qualities that make the player an NHL and have to determine if they can overcome their weakness(es) to make it to the NHL full time.
Coleman is basically prime Burrows on a contract that pays him nothing. It's not really correct to classify him as a '3rd liner'.
Most bottom-6 NHLers were drafted as skill players, couldn't make it on their skill, and then found ways to adapt. Gritty CHLers who 'project as good 4th liners' almost always end up in the ECHL. Pat Maroon who has been talked about here was a huge junior scorer.
When you look at the 'hits' in the draft after the 3rd round, they virtually all fit into a few categories :
1) Talented guys who fell because they were small.
2) Talented guys who fell because they were slow (and this might be the unfortunately named Kunz).
3) Talented guys who slipped through the cracks due to lack of exposure/playing in a weird league.
4) Talented guys who fell because they had been passed over in a previous draft and were late bloomers.