VMart is not a disaster behind the plate, and actually was showing signs of improvement before his injury. In 2007 Martinez was markedly better than Varitek in several key areas, especially CS%, and he was tied with Joe Mauer for second in the American League in fielding Win Shares for a catcher.
As for why they'd give him up, the reason is pretty simple -- Kelly Shoppach has arrived, is, for now, cheaper, and has more defensive tools and significantly more power than VMart. I dunno if the Guardians GM even seriously considered it, but it's the move to make IMHO for the Guardians, especially if they can get a good SP out of it. They have a rotation right now of Carmona, Lee, nada, nada, and nada.