I think it depends on your roster size. Last year our league bumped it up to 23. With that size, you have the luxury of carrying 2 catchers.
As a principle, I think it's smart to carry 2, because there are so many games that catchers sit out. The easiets situation is if you have Varitek ... always sits w/ Wakefield, and you plan on inserting catcher #2 into your lineup that day. Same w/ Barrett and Maddux.
It just kills me to have an empty spot in my lineup, with a guarantee of no production.