There are advantages to fishing a centrepin-type rig with a fly line. It's still fly fishing despite what the dry fly elitist argument, not that that is what symmetre is saying. You can roll cast accurately with practice and then mend, or even mend before your rig lands depending on how much weight you have on. You can pick sections of water behind rocks or structure that you wouldn't necessarily be able to access from an upstream float. Another advantage is you can switch up your method as you encounter different water on the river. Switch from a float type method to a streamer or woolly bugger if you see a deep seam, etc. You can even get good at "walking the dog" as you float, which means you let out line as your float drifts away, similar to a centrepin letting off line, although it's not as effective and you can't set quite as well. Point is, you can still be very effective "float" fishing with fly gear.
Don't have anything to add for flies, but another tip: changing weight and position of it is better than changing flies.
