If you don't have any assault weapons to fill in your grid, then keeping your copies unbroken is fine, but DON'T level more than one! If you choose to have fire as a main, eventually you'll have a bunch of 3* SRs from the Fire Disastre, and a 3* SR is way better than a 0* Shikigami Fire gun... so eventually you'll want to have a 3* SSR. Also, note that when you use something to break limit something else, any exp/skill xp is WASTED. Meaning if you have a 0* SSR at skill 20, and it's used to break another SSR, those 20 skill levels are gone; completely wasted.

TL;DR: You can use them temporarily, but don't level up the skills, as you'll eventually use them to MBL one of them.