Except for a once-in-a-generation talent like Phelps, Spitz or Summer Sanders swimmers don't get that many medal chances.
A swimmer like Amanda Beard would compete in 2 (100 & 200 breaststroke) events and a relay (Medley), Janet Evans was a 2 event swimmer (400 & 1500 freestyle) with no relay, Gary Hall or Alexander Popov were 2 event swimmers (50 & 100 freestyle) with 2 relays (200 & 400 freestyle), Ryan Lochte and Aaron Piersol were backstrokers.
In all reality, backstrokers tend to be backstrokers, breaststrokers tend to be breaststrokers, sprint freestylists tend to do sprint freestyle, distance swimmers swim only distance, and butterflyers tend to be well-rounded.
Actually, if you think about it Spitz, Sanders, and Phelps were all butterflyers.