Ironically, I can see this as a weapon for the pyro and medic as a flamethrower and syringe gun, respectively. Just imagine the pyro coming at you with burning piss, or the medic using it as a weapon that he can use to support his team by sprying jarate on them. Heck, the medic could use it similar to the crossbow, giving a slow measured healing when used on his allies and adding jarate to his opponents.

The ideas are both interesting and disturbing at the same time:
Sniper: Here you go, medic! A weapon for ya.
Medic: Is... is zat your pee in zat bottle!?
Sniper: That's right. Just the thing to help our team!