[quote]why don't we use both of those currencies?[/quote]

I feel like it would be too difficult, why use two when we can use just one. Another reason for example, Diamond at the very least has a very large sum of lien from past criminal activities, it's be a bit unfair. I'd think of it that the school itself would only allow you to get a weapon upgrade after reaching certain limit of credits. 

Lore wise, unless something drastic happens to your character that would change them forever, anything but a light semblance upgrade is pretty much a no go, and even that should be something you work the better part of the semester for.

And Lug, as far as buying subplots goes... Ain't that a bit too limiting? We have couple dozen people, not couple hundred, and most of thsoe are grouped into packages of 4 for any of the given subplots...