Glen's biggest asset is his health, honestly. If he weren't counted on for being a reliable backup big amidst a group of unreliable (health-wise) group, then he'd probably have been gone by now.
I don't see Glen being traded for this very reason, and I think Danny will likely let him walk at the end of the season. I really doubt he'd net too much in return in a trade, so it's better to have him as insurance in the playoffs than try to eek out something in return via trade.