Personally, I'm of the opinion the #28 should not be worn again. Maybe not retired, but kind of have a thing going on like #14 does. I still remember when the Flames traded Robyn Regehr for Chris Butler, and Butler was originally given #28, and he voiced his objection to it, thinking it not a great idea. He was probably right.
A couple of years later, though, it appeared again, if only briefly. And again, briefly. And now Emile Poirier has it. He still has two years left on his entry level deal, so he'll probably keep that number a little longer yet - and he's still got plenty of time to prove he's earned it instead of his initial training camp #57.