Scarcity is important, but catchers tend to be overpaid for. I think that at this point in the year, Yahoo Ranks (not O-Ranks) are pretty accurate. Mauer is 99, V-Mart is 174, and there are 8 in the 200-300 range. With all of those second-teir catchers clumped together, why overpay for Mauer of V-Mart? Just don't wait too long and get stuck with a totally crappy catcher.
I took Ramon Hernandez in one of the last rounds, and while I'll admit that his production has been a least a little bit lucky, that didn't turn out to bad. Catchers aren't worthless. Mauer is worth more than Ramon, of course, but is he worth like 10 round more? I doubt it, but it'll probably happen.