Hmmmm....... I have a Sage 6wt, and it's a fine rod for tossing bigger stuff in stiff winds. I also have a 5wt Sage rod that will toss bigger stuff better than the 6wt, so it depends on the rod. I ran into a guy fishing Ridley Creek, and he swore by his 6wt for trout. He said that he needed it to turn the fish. Having caught several large (16+ inches) trout and smallmouth on a 4wt, he has a point. I've caught a few 16 and 17 inch smallmouth on my 4wt, and for the most part it was fine. One 16 incher was caught while fishing for trout last year. I was armed with the 4wt and a #18 BWO on 5X tippet. I saw smallmouth tearing through a school of minnows not long after arriving at the stream's edge, and switched over to a 3X leader with a Black Nose Dace. It only took two casts to hook into a beast of a smallmouth, and the fight was on! This section of the stream is wide, and has a featureless bottom. All I had to do was hang on for the ride, and was able to land the smallmouth in less than 5 minutes. He took a wild run downstream when first hooked, however: I left the drag set for the lighter 5X tippet!
I was fishing another section of the same stream a day before the last hurricane hit SE PA. I was armed with my 4wt, and a #12 Wooly Bugger. This section is at least 100 feet wide, and funnels into a stretch that's 20-25 feet wide. The bottom is strewn with a zillion large rocks and boulders, and fish tend to dive deep and look for cover. I hadn't expected to catch anything too big that day, and just needed to get my fix in before the stream flooded.
I was wading close to the narrow section of the stream, casting
up toward the much wider section. After only a few casts, my Bugger stopped and it was on! I should have moved away from the faster current, but I didn't. After many ferocious runs and dives for the bottom of a deep channel, I was able to get the fish in close enough to see that this was the biggest smallmouth I might ever catch in my life! That's when he saw the current, and made its move. I tried to turn the fish, but it was just too much weight on the rod. With the fish just a few feet from the raging current, I decided to slam the brakes on the reel. He leaped like a tarpon, and revealed himself to be at least 20 inches and FAT!
When he slammed down, the hook popped out, and he was off.
If I had a heavier rod, I could have turned that beast, but the fight was worth being there that day. I don't think my heart has ever pounded so hard while fishing!
I hooked a 16" brown a couple years ago with the same 4wt rod, and he just dove for the bottom (more like a smallmouth than a trout). Fortunately, the stream bed offered him little to wrap around, and it just took time. I've lately been thinking that fishing bigger waters are fine for lighter rods, while water like the stream with the large smallmouth might be better suited for something with a little backbone, especially of you don't know whether you'll hook a trout or smallmouth, and whether it might be the fish of sa lifetime. I certainly don't see any disadvatange to using a 6wt rod for trout. I can slapthe water just as hard with a 4wt line as a 6wt, and land a 6wt line almost as delicately as a 4wt. ;-)