The popularity and demand of fiberglass rods has really skyrocketed in recent years, and fly rod manufacturers have responded with an influx of new models. Fly anglers are loving the looks, feel, and retro vibe that glass rods provide, with some folks seeming to avoid graphite rods altogether. Graphite will always be the king of versatility as far as I'm concerned, but fiberglass definitely has its place. After publishing a few fiberglass rod reviews here on the site, I thought about 3 solid reasons you might want to consider fishing with a fiberglass fly rod....
For a Slower, More Relaxing Action: Fiberglass fly rods are noticeably softer than their graphite counterparts. If you prefer a more moderate to moderate-fast graphite fly rod, a glass rod might be a great fit for you. My casting stroke favors a faster rod so the adjustment to glass isn't so seamless, but if you already fish softer rods you'll likely adapt much faster. Now, there are some glass rods out there that are marketed with a faster action (like ECHO's "Glass Fast" specification), but you'll still notice the distinct difference in flex and responsiveness when compared to a fast graphite fly rod. Still, some glass fly rods can be surprisingly capable even with larger flies.
For Delicate Scenarios: If I lived where I had small trout streams nearby, you can bet I'd own a little fiberglass rod or two specifically for those spots. Glass rods are buttery soft and bend deep into the butt section with a good fish on, thus making them ideal for fishing with lightweight tippets and small flies. That soft blank will provide ample cushion to keep light line from breaking and keep tiny hooks from pulling out.
For a Totally Different Feel: Catching fish on any rod provides some measure of fun, but fiberglass can amplify that fun factor. Even if you're just catching the same fish as always, the way the rod bends and the totally different feel it provides can really provide a whole new experience to the angler. Until you've felt a glass rod bending and bouncing with a fish on, it's hard to fully understand. Do you encounter a lot of small fish? They feel much bigger on glass!
As eluded to earlier, glass rods may not be for everyone. While they can be thicker, heavier, softer, and less sensitive when put up against an equal graphite rod, I think in this realm those things don't matter so much. It's all about a whole new feel and experience; one that's more raw, retro, and simplistic.