When should I use ministorage?

Normally never. If you own so much that you do not have room, the things that go into storage are usually never used, and better sold or given away. If perhaps you are moving or going to be without a home, use it temporary.

But more often, we store junk that we seldom use, and pay unneeded fees

When you have no room at your living quarters to store all your possessions.

?
When it suits your needs and are willing to pay for it.
Not sure what you are asking, nor what defines mini. I rented a 10 x 15 bin between selling one house and buying another.