Why would anyone use the Degreaser? Faster weapon switch? Who cares?
Believe it or not, it's the reason Pyro can easily be seen as one of the most overpowered classes, as Freeze said, those combos allow 1 hit kills, it doesn't work with other flamethrowers due to the delay in switching.
The axtinguisher's crit hits 195 on enemies on fire, so light them up, and switch within the same second and kill any class besides Heavy which takes 2 hits.
I'm actually very surprised they haven't nerfed the thing to hell.