Well, I think SOME benefit of the doubt has to be given to Koekkoek in that TB's Left Side Defense is probably the hardest position in the league to crack. He's behind Hedman, McDonagh, and Coburn, the latter of whom is playing like he's 25 somehow. There are defensemen a lot better than Koekkoek who wouldn't be able to crack that left side at the moment.
Cernak forced his way onto the lineup, but he had a much easier path, being a righty. Our right side is Stralman, who was injured for about 15 games, thus giving Cernak his shot, Sergachev, who struggled in the early portion of the season, and Dan Girardi. Cernak might legitimately be better than Girardi, and yet we still rotate them. The math against Koekkoek is about 10x harder.
Again, that's not to say Koekkoek is a sure-fire NHLer, lost in the mire. It's just that we really don't know. He's performed well and performed poorly at different stages, and been roadblocked either way.
All that said, he might be one of the safer reclamation projects on the market. He could be had fairly cheap (a mid-rounder in season, less in the off-season), could probably play in most NHL lineups tomorrow, and still has a pretty good upside. Not quite the risk-reward scenario of say a Justin Shultz, but, still, teams who need a warm body could do worse.