I think I might be the only one who prefer using the RS on Pyro, it seems. I have a couple reasons for using it over the stock shotgun...
1. It's the best Anti-Pyro weapon available, especially when it comes to facing Phlog Pyros. Two good airshots and they're dead, and most Phlog Pyros are either too stupid to swtich to a secondary or carry the Manmelter, so they're a sitting duck.
2. One good airblast reposition and airshot can kill Scouts harassing you. Scouts that are always jumping around, especially when doing something like carrying the intel, are easy prey if caught unaware as well.
3. I tend to anticipate when an enemy will be jumping (say, making a leap for the water to extinguish fire) and put a stop to their retreat with it. The minicrits can mean the difference between life and death for them. Especially effective on enemies who you've damaged enough to force the retreat yourself, in the occasion that you keep whiffing with axe swings.
Besides, it's always just such a satisfying combination to kill people with. :3