Why is it people have a problem with cure minor wounds working at will?, It would take 100 rounds(so 10 minutes) in order to heal a low level fighter from 0 HP back to his full(or likely slightly less than full)HP, any good DM won't let players rest that easy in a combat area, such as a building being raided by zombies, or a graveyard being raided by zombies., And out of a combat area it makes sense to heal up, and the PCs are likely to heal back to full regardless of a low level spell being at will, and will likely simply use a higher level spell, and then sleep.