The 5-weight has long been known as one of the most popular rod sizes in the world of fly fishing. This popularity stems mostly from the trout fishing realm, as this rod size is a great one-size-fits-most choice for general nymphing, smaller streamers, and dry fly fishing. While the 5-weight is indeed a must-have for most folks that chase trout, in my eyes there's just so much more versatility with this line size.
Over the years, manufacturers typically started equipping fly rods with fighting butts starting at the 6-weight size. Often, two different 6-weights are offered; one model with a fighting butt (often referred to as a "saltwater 6 weight"), and one without. Keep in mind I'm talking standard 9-foot rods here....fighting-butt equipped 6-weights can also come in long 10-foot lengths for specialized nymphing or light summer steelhead applications.
In the last several years, I've noticed several companies changing things up by selling 5-weights equipped with fighting butts and full aluminum reel seats. Off the top of my head, current offerings include the Redington PREDATOR, Sage X, EDGE Gamma Beta, and Clutch Archipelago. For the record, I've reviewed all but the X here on Demystifly, and those three were all exceptional rods.
Why would you want a 5-weight with a fighting butt?
I regularly use my personal fighting-butt equipped 5-weight with tippets up to 12-pound for species like snook, largemouth, peacock bass, carp, and streamer fishing for larger trout. While a standard 5-weight can be awkward and uncomfortable for longer battles with strong fish, a fighting butt simply makes for a MUCH better fish-fighting rod and turns a 5-weight into a true light-tackle weapon for both fresh AND saltwater. Once hooked up, just stick that fighting butt in your gut and enjoy superb control and easy reel crankin'.
A fast action 5 with a fighting butt is a surprisingly capable rod and can toss some decently-sized flies provided you choose the right line and the wind isn't an issue. Not only is a lighter fly rod simply more fun to fish, but if faced with calm, shallow waters, it can also lead to better, less-intrusive presentations. I've caught fish up to about 20-pounds on 5-weights and my only worry during those times was if the line would hold or the hook would pull. With sound technique, you can efficiently whup a big fish with one of these rods. I will NEVER be without a 5-weight with a fighting butt.....it's simply too useful and fun.