If your central argument is a small sample size, increase the sample size and take a look at his stats in the 200+ games with Canes. He's got better stats when compared to, for instance, Justin Faulk or Tim Gleason and he's on par with most other defensemen.
He's not an elite player, but he's a decent bottom-pairing guy whose contribution this past season hasn't been as horrendous as one would assume judging by this thread.
He's not exactly going to create a gaping hole in the Sabres defense, but the overall quality of the D corps has decreased with him leaving, unless the plan is to spend money in FA and sign not one, but a few defensemen.
If that's not the case, with Ehrhoff/McBain gone (and McNabb, btw) the Sabres are left with Pysyk/McCabe/Ristolainen as the alternatives to the role Ehrhoff and McBain played on this team. And if you compare their stats and disregard their age or potential, it's pretty obvious that this move would only work in favor of the tank.