Well, I don't completely agree, because I really think Piazza *is* worth a 1st round pick, maybe 2nd round in very shallow leagues (like a 10-team mixed league). He's a consistent .310-40-100 hitter, which is really as good as they come for catchers (bonus points if anyone can tell me the ML record for HR's by a catcher, set in 1996). No other catcher in the NL (I'm not really up on the AL, sorry - and IRod is real good, but look where he's at now) was projected to have more than about 20 HR's. With the exception of Jason Kendall, who can give you some SB's from the catcher position, no other catcher is really worth even *half* of Piazza's value. Paul Lo Duca can be expected to hit maybe .290-20-75, and he's about the third best catcher in the NL behind Kendall and Piazza.
In a "standard" league (16 team NL or 14 team AL?), the last team to pick a starting catcher is left way out in the cold. Even the 5th person to pick a catcher starts noticing a general lack of options. For outfielders, you can drop quite a few positions before you really feel the hurt. Sure, Sosa, Bonds, Vlad, and the like are all the top of the totem pole, which is why they are generally 1st and 2nd round picks as well, but when you drop to the Chipper Joneses and the Brian Gileses, you don't lose nearly as much as you lose dropping from the Mike Piazzas to the Mike Lieberthals.
These *are* the same reasons why Roberto Alomar and Alex Rodriguez are so highly prized, so you can't completely discount position scarcity. It's extremely hard to get that kind of production out of their position. But the catching position is the only one where missing the boat on the top stars can mean so much.
BTW, when I say that I don't really believe in position scarcity, I mean that I don't believe in sacrificing *too* much for the sake of drafting to position scarcity. I'd almost certainly draft Sosa above Piazza, because Sosa has basically the same potential, plus 20-30 homers, and 30-50 RBI's. Sosa is like a Piazza and a half. But when the numbers get close, you gotta go for the catchers and the middle infielders first.