@Myrdin, your point on Minnie/Katyusha is valid, which is why I generally use Minnie in two situations:
A) Boss killing
B) Deathball wrecking when I do NOT have multiple medium tanks/TDs out (since then you end up wasting half her skill on enemies killed by your other tanks). Pop her behind a heavy and let her rip, basically. Usually kills enough to let me get a few mediums/destroyers out and build up deathball.

Whether the above uses validate using both is debatable.