Interesting thread. Fly rods are like golf clubs, you need a different one for every situation. No golfer would ever play a rou d with one club.
I have them from 3wt through 11wt.
I generally prefer a fast action, but all around for trout fishing a medium or medium fast action rod is more versatile since they are much more pleasant to throw weighted nymphs and strike indicators with.
A slow action is usually easier for a beginner to cast with since they empathize the feeling of the rod loading and help with a novice's timing - and timing is everything as experienced guys know. I do not like the feel of a slow action rod for casting though, and do not like the feel fighting fish either.
Also, remember a fast action can be slowed by over lining it, and that is one way to allow a rod to suit two situations very well, rather than one really well and the other make do. For example, slapping hoppers or crickets across the stream to the far bank - best with a fast action, and casting a pair of nymphs , maybe some weight and a strike indicator upstream a few yards - best with a medium action. Just carry an extra spool with the heavier weight line, with maybe a different taper to the head.
A five wright can be fine for smallmouths, as long as you stick with wooly bugger, crawfish and helge mite type flys. For some of the leach patterns a 7 weight ok and an 8 better. For some of the bigger Dahlberg inspired creations I use a 9. A medium or medium fast rod for the weighted stuff, but for the big and bulky hair bugs, frogs and "minnows" I like fast or faster.
For saltwater, fast or faster.
Years ago budget fly rods just didn't cut it. But today's budget fly rods are often very good rod blanks with less expensive fitting, guides, etc. Most seem to be medium or medium fast, which makes them pretty versatile. A fellow can outfit himself very well without spending a fortune.
My 2 cents.
JPK