You can be sure that there's a system in place to prohibit players from having every aura that they've acquired in effect -- it's called your "concentration" attribute. The higher your concentration, the more auras you can have active (among other things -- certain powerful weapons, for instance, also have concentration costs). Higher level versions of certain auras will cost more activate than their lower level counterparts, which serves a dual purpose. First, it guarantees that higher level players can't "twink" their friends with super-powerful abilities, due to the prohibitive activation cost. Secondly, it guarantees that lower-level versions of abilities remain viable throughout the life of your character. You may not want to, say, equip your highest fire-resistance aura, but one a few notches down will be significantly cheaper, while still affording a satisfactory degree of protection.​