what I find greatly ironic, is that people complain that the sniper is not a team player as it were, and he just stays at the back and pics off people, with them having little to no chance to attack the sniper, thus meaning they can't survive well, to combat this problem, valve make more close range weapons and team based weapons, e.g. the huntsman and jarate.
this makes the sniper a more friendly support class. rather than just picking off people from a distance, he can become more of a team player, by putting out fire, or, making it easier to kill a hard target, e.g. a heavy. (Which will go down in about 4 rather than 5 rockets, if jarate'd).
Before this update, the all the support classes had atleast two roles. medics were able to heal and supply ubers. spys were able to pick off the heavier classes, as well as slip into the enemy team and provide recon (Especally useful in clan matches more so than pubs). the sniper however, just stayed at the back and picked off people, which could have been done by the spy.
the update also makes the sniper more viable to play, there are only a few select places one can play sniper in valve maps, which pretty much makes the sniper a unplayable class in some maps, or atleast restricts him to one or two places. and this becomes especally hard for new players to get into that class. with this update, the sniper becomes more viable for other places. he is more free to move, with the one second charge up time, he could release a bow, at close range, and then flee back for cover, or he could throw a jarate at a random enemy and then retreat back.
it may be a gimmicy weapon, but it does it's job. which is to make the sniper much more of a team player. the same thing for the boink energy drink; which makes the scout that bit more viable on Attack Defend maps when there is 10 sentries on the enemy team.
besides if you think a jar of piss is immature, then that's just pretty stupid, team fortress 2 is not a game that is ment to be taken in a serious nature; if it was, then they wouldn't make maps the way they are. tf2 is supposed to have that small bit of immaturity, which is what gives the game it's character. otherwise it'd just be another CS:S/COD/Other FPS.
Final point: I'd be pretty sure urine is used as a weapon, and I think it is able to blind people (Which I'm pretty sure you'd rather not have that as an effect for the jarate). plus the weapon does fit in with the update in a way, tribes people always try to maximise the useage out of what the "harvest" (Couldn't think of a better word here), so it would make sense for them to use urine for something.