In our recently finished campaign we removed the restriction on permanent AC. We reintroduced it after a year of in-game time when we were starting to need a warehouse to store permanent AC to half the places and important people of Europe. It became silly really fast. Limiting AC creation to the amount of vim vis you can use is effectively no limit at all. What we did is allow more leverage on the amount of free time you had. You could still lose a month of lab time (going to tribunal for example) and fix an AC. It was done ad hoc, not as a hard rule, but it worked OK. The current official system is there to limit hundreds of ACs to be stored one after the other, I thiunk, so it is stuff regarding game balance (and above all, suspension of disbelief).
An AC is always an important thing in the stories, not a trinket. Allowing multiple ACs to be fixed per season devalues the ACs as a whole.
Xavi