Suppose at 3.0 upon launch you have all your jobs at 50, including DoH/DoL. You unlock the 3 new jobs.That's 20 rings.
- Tank Lv 50 rings x2
- Healer Lv 50 rings x2
- STR Lv 50 rings x2
- DEX Lv 50 rings x2
- INT Lv 50 rings x2
- DoH Lv 50 rings x2
- DoL Lv 50 rings x2
- VIT Lv 30 rings x2
- MND Lv 30 rings x2
- DEX Lv 30 rings x2
Now you level your main job up to 60. There goes 2 extra rings. You level your second job to 60, another 2 extra rings. When you want to level your 3rd job to 60...guess what. You don't have enough space to carry all the rings you need.
Now mix in DoL/DoH at different levels. And there's a real need to have a bigger AC, at least for rings.