After awhile they become a joke. If you doubt me, look at the comments here. People are laughing at the elevated alert because they don't mean anything. There is nothing specific attached to them. I think they are stupid because of their vagueness. You know darn well that who ever pushed that they be elevated had something specific in mind.
Surely they don't think we'll become hysterical or something. You know, if that is the situation, - when you treat someone like a child they act like one after a while.
The purpose of the alerts is mainly bureaucratic CYA.
If something does happen, they can say "We warned you!"
Never mind the warning was essentially meaningless because of its total lack of specificity.
The one possible real benefit is that it raises peoples awareness of a threat, so they may be more aware of some unusual activity that they see and may be more likely to report it