Let's say you have an FC that does Coil, and requires you to have multiple jobs at 50 because the FC has different people that log on, like back ups and what not, they require this so you can be flexible. You have your main job geared rather well, but your other jobs not so much, you consider those your secondary mains and whatnot. But you under-perform ever so slightly because of your gear or lack thereof. You start to hate the system and wonder why there wasn't a cap per job that fills from gaining the cap with just one. Imagine you did all your AK/WP for the week, you have 300 myth tomes per job you have at 50, and can only buy gear for said job while on said job.

Now you can get gear for your alt jobs instead of taking another 5 months to gear it when it becomes outdated. That's the underlined issue, by the time you attempt to gear that second main it is outdated. The system I mentioned above should be in place. There, saved frustration. It's not a matter of I WANT THIS NOW, it's a matter of I want this before t becomes useless on my second main.