Kat is a must to deal with blobs and statics positions (mgs, bunkers, mortars). These are 100% things you will find in any match.
No it is not a "must have" it is not even that good at it. Katys are used best for stomping those pesky 4 squad lmg gren squads or tripple sniper builds, that often only gets dangerous when pooled with good armour.
Su-76 are a more effective way of knocking mgs and bunkers.
M5 will deal with mindless A-moving grenblobs (advance with a pak and your golden) Su-76 will break them up. However that goes out the window in late game, with Panthers and such.
Only then will you need the Katy, if the grenblob is still on the field your M5 will be knocked in the next attack, with the grens supported by a Panther and instead the Katy will take on the roll of deblobbing the enemy. Whilst your SU-85 deals with the Panthers.
You have now a soviet faction with early, mid and lategame, I suggest you combine all three for best result.