I guess "intended species" is a good thing to consider, but honestly I probably lean more toward "intended flies and conditions" now.
I like heavy woolly buggers, clousers, and bulky poppers on occasion. I may catch mostly smaller bass/catfish about 90% of the time...but big flies are a lot of work on a 5/6 weight. The 8 weight does it with ease.
Interesting opinions -- everyone is a bit different on this it seems