I think it can be helpful in the following circumstance:

When you're going to change the fluids thoroughly, add a can the day before. It CAN help reduce sludge and gum buildup on the inside of a transmission. Whether or not this is a good thing for an old transmission is open to interpretation. But if you've got one on its last legs, or you don't know the last time the fluids have been changed, I say go for it.

Leaving a can in there... I don't want that stuff acting long term on the seals.