Hey guys I have a quick question. I am a long time user with android and rooting and flashing isnt anything new to me, but I have a question on something that is the first time to happen to me.

When I sometimes go and flash a rom it fails. I go and try to wipe dalvik and cache, and system and it says cant mount system. All in red letters in TWRP 2.8.0.3, I have to reformat to fix the issue.
First question: Is it bad to format everytime I flash? should I do it? I normally dont until I notice the error to mount system. It happened when i was going from sense based rom to aosp. I am on the updated 4.4.4. I asked this on xda with no answers. thanks

2nd question: does the error system mean that the internal card is dying?