Why upgrade unless you need a new feature found in the new version? To me the only reasons to upgrade to 5.8.0 would be the dire need of the New IO Implementation features, New Thread Implementation (stable threads?) or Better Numeric Accuracy. Or I guess to fix a bug that there is no known or acceptable workaround for in an older version...
Well, one possible reason would be that you are a module author and want to test your modules against 5.8.0 before publishing them. Especially aginst a threaded 5.8.0 perl...
I like the new I/O features, since I'm working a lot with Unicode these days. That made me install 5.8.0 alongside 5.6.1