I believe that the abundance of PrCs are becoming a problem simply because for someone look to do something, there are too many options. From the point of view of a DM I tend to keep some PrCs on a low down merely because I restrict the use of some books(not unfairly of course, and never 'just because'). I, for example, don't generally let my players use the Book of Vile Darkness/Exalted Deed and the Frostburn/Sandstorm style books. Those I consider mainly for DM use.. Of course my players tend to go insane over the really powerful and excessive stuff. All things considered - it's a good thing for the market because people keep on buying them anyway, whether or not there's too many or too little. For me as a PC, the prestige class often helps refine my character (from a 'fighter' to a 'leader') and give a more specific flavour. I consider prestige classes that I could play with my characters and not a character to fit a prestige class as some do.