EDIT. On longer thought - I think of basic arrow management, the same way as weapon breaking in BG1. I thought it was cool, but gameplay wise didn't contribute much, and didn't miss it in BG2, were either system is completely abandoned, or because most weapons are magic weapons, I never run into it.
Still, it is one way to make progressions - from having to manage arrows manually, and carrying a spare basic weapon or two, to using this space for you default xxx-Of-Plenty, and carrying special arrow types and various unique magic weapons for various situations.
