Mini sentries are bad in MvM because:
1) 500 dollars is usually the payout for a normal round if you've gotten an A, that really is way too costly in terms of its actually utility.
2) 500 can and should be used towards the more essential upgrades; sentry health, firing speed, more metal for you, or wrench speed.
3) Sentry firing speed will deplete the mini sentry obscenely fast and you can't refill it. So you basically have to toss it down in a spot where it will die, which takes time/you away from your main sentry/dispenser etc. You might say oh but I wouldn't leave my sentry etc, great you have a 500 dollar upgrade going click click click.
In normal pvp matches I do use the mini sentry when playing an offensive engineer. I drop an entrance at the start and then push out with my mini sentry and shotgun to secure a forward point for my allies to warp in and in general cause some havoc while everyone else is focusing the usual threats.
NameIsRobertPaulson said:
That said, on MvM, STOP USING WRANGLER. Spend that time fixing your sentry rather than wasting half your ammo trying for kills. Seriously. Had a guy claim I was hacking because my sentry had 27 kills, and his had 9. No, I wasn't hacking, I was fixing the gun while it chewed through the enemy, while you sprayed rockets at nothing in particular before having yours destroyed. A max durability upgrade means that sentry takes FOREVER to kill.
I'm sorry but the wrangler is absolutely amazing in MvM. It doubles the firing speed of your sentry and stops 66% of incoming damage. Granted for normal waves it really is best to let the thing auto fire while you spend your time repairing and what not. But when it comes to last ditch omg there is a tank or a super robot that is blowing people up, dropping a level 3 sentry and wrangling it puts out more dps and the shield can actually tank a super robot. On more than one occasion I've beating a super soldier or a super heavy with just a wrangled sentry.
It can also pick off those snipers in MvM who will take you out if left alone.
And even then is there really any better alternative? You have the choice of the pistol which you won't use, or the short circuit which is finiky at best.