If you don't see the skill Kassian has in cycle, in moves around the net, in scoring acumen the rare time he gets the chance I can't help you. This is a player that doesn't look out of place in topsix. The trouble is, this same player, with hands, does the least possible work on any other line.
Kass is a 13th pick. Not some no talent slouch. His combination of size and modicum of skill was thought to translate to some success at this level. He has tons of advantages. Reasonable skating, size, strength and decent hands. He reminds me of Pouliot in the sense that he has so many assets and doesn't use them. Kassian can really elevate his play when he wants to.
He was meant to be a rich-man's Moreau. Playing a heavy game nightly is hard, not everybody has the heart for it.
That's a great reason to get rid of him... similarly, it's a great reason to keep him around for the playoffs (assuming we had the other horses to get you there).
Reider? I'm not sure which camp he fits into between A) regular season performers to get us over the hump, or B) guys who lack consistency, but can amp it up in big games. Does he fit in either? I don't think so.
Kassian? Meh... I see a guy who can rally the troops occasionally with a big hit and a timely goal. The guys seem to feed on his energy. The problem is he's not that player most nights... so in that sense he's part of the problem.
I'd much rather have Kassian than Lucic, but given we're stuck with one of them, the other becomes somewhat redundant.
As for Reider... flush him. An energy skater/utility guy with questionable finish can be be had much younger and cheaper (and who knows, perhaps still possessing some upside).