I've been running with no catcher in my 12-team mixed roto for the past week and a half. I've got Piazza on the DL, and am giving serious consideration to continuing on without a backstop until he's activated.
I initially used the roster spot to pick up Slowey, whom I've subsequently dropped for Homer Bailey. I figure I have a few weeks to negotiate a two-for-one trade before Piazza returns, and in the long run, can end up helping my team more than a second-rate catcher ever would have.
(Hopefully that all makes sense...)