It's not even about verisimilitude. Resource management is an important part of most games and RPGs amongst these genres where this aspect can enhance gameplay a lot. Not as weighty as in strategies and economical games of course but trying to save your ammo, mana/spells/scrolls for next encounters, avoiding too much damage so you won't run out of potions/first aid kits or will be able to get to next safe location to rest, managing space/weight of your inventory and deciding how to spend your hard earned money can provide very interesting challenge. It can also make the game much more exciting - there's nothing better than beating especially difficult opponent with one of your last bullets/spells/with a sword that is almost broken.
Using infinite ammo in an RPG/shooter hybrid would quite strange decision. You take one genre that is historically very light on resource management and other that involves quite a lot of it and then you don't try to find any compromise but take away even the little management element characteristic for shooters. Makes no sense.