True it does cast smoother, but it does do some damage to your line guides and your reel, so you gotta weight the benefits. The thing with mono is it's guaranteed to acquire memory. Under normal circumstances, I find my mono is only good for about two trips throwing spinners before the need to respool, but that's personal preference/lack of tolerance to put up with it. They do have some stuff called "reel magic' or something like that, that you can spray onto the mono on your spool when you first respool and it reduces the memory, I've had it sprayed onto my spool when I bought a new reel and the shop included a free spooling, it did make quite a difference.
Also, using lighter mono, increases your manageablity. use the lightest line you think you can get away with. You don't need 20 pound line if you're not chasing salmon. Heaviest line I use for steelhead is 12, with either a 6, 8, or 10 pound leader, depending on water conditions. Trout get the 6 pound mainline, 4 pound leader treatment.