Okay, just to leave my input here...
I use the Reserve Shooter on Pyro because it's SO much easier to get a kill with it than the flare gun. Sure, if you're pro at aiming with the flare gun, you can use that, but the Reserve Shooter is a good "crutch" weapon. In case you didn't realize, using the reserve shooter is exactly like using the flare gun. You airblast them up, whip out the gun, and fire for mini crits. This is SO much easier at destroying enemies than the flare gun with its kinda-difficult-to-aim crits.
Just my opinion, of course. Many other pyros are great with the flare gun or detonator, but I find the Reserve Shooter more my forte.
My usual loadout is stock + Reserve Shooter + Homewrecker for a very support and defensive Pyro class.