While it's certainly helpful that you regenerate ammo, there's a few reasons why you should buy ammo capacity upgrades.
- You won't regenerate ammo as quickly as you may think while gunzerking.
- You only regenerate ammo outside of a gunzerk if you're specced to do so.
- You can only regenerate as much ammo as you can hold.
- Eridium is not only plentiful, but almost worthless once you start TVHM. Just upgrade all SDUs up to Blue in playthrough 1, and move on to maxing them out in PT2. You'll have max capacity by level 45.