Well, saying it's the weakest revolver in the game isn't quite fair. The weakest in MvM? Sure. But plenty of weapons are really weak in certain situations. Natasha sucks everywhere but MvM. Pretty much the same with the Beggar's Bazooka. The Neon Annihilator is a straight downgrade to the Fire Axe if playing on a map with no water, having to rely on scouts/snipers to get enemies wet.
There are plenty of situations where the Diamondback can be an incredibly potent revolver. It's just not while playing MvM.
Sorry for lack of proper wording, when I said game I did mean MvM. And you are right in that certain weapons depend on the mode and group, just like classes.
I wouldn't say that the Natascha sucks everywhere but MvM, I've seen and used it effectively in CtF and even in basic Attack/Defense games. The ability to slow down people long enough for say, a Solder's Liberty Launcher shot to hit a small cluster of people or to allow the Sniper that split second more time to line up a headshot can be invaluable. Not my personal favorite Mini, but seeing Scouts and Demoknights ragequit is just hilarious to me under most cirumstances as well.
Can't say much for the Beggar's Bazooka, but I've seen it used an awful lot in "for fun" game maps. People seem to do ok there, and in a few Saxton Hale servers I go to too. I don't usually play a Soldier anyway, and I'm really unorthodox when I do (My set usually consists of a Rocket Jumper, Mantreads, and Market Gardener).
Neon Annihilator I can see your point on one hand, but kind of not on the other. Either you need water or something to make the opponent wet, which is a bit random. On the flip side, it is rare that I see a Sniper that isn't packing Jarate and the Mad Milk seems to have gotten more fans in other modes since MvM came out. It can also be a slightly handy alternative to the Maul/Homewrecker since it can detach Sappers, but deal a little more damage to players (Minus the extra damage to buildings [Because Robots don't count as buildings, this can make the Neon better in MvM] and the lack of random crits) I personally use a Strange Sydney Sleeper, which makes a Pyro using buddy of mine smile when we play. But, that is beside the point and I concede this one since it is a really good example.
But, not sure how this turned into a case of arguing over when and where a weapon is useful or not. Just got a little confused as to why they didn't make a sapped robot give a crit. Nothing game breaking, just confusing. Not that I really care anyway, I use my Strange L'Entranger under most circumstances when I play Spy. I'm more holding out for them to fix a lot of the issues that seem to be coming up regarding getting into the game and staying there, a lot of MvM servers I was on today crashed at the exact same point.
Edit: Had to fix some grammar errors and make this thing actually sound like a list of reasons instead of a ramble.