The flag warning is shown to a user based on flag statistics within a window of 7 days. This means that flag warnings, and by extension flag bans, are optimized for users who flag a lot.
Besides the flag warning, the only other way for a user to access their flag history via the UI is through their profile page. Users who care about their flag stats enough to take full advantage of gamification will figure this out sooner or later, but these users probably won't:
Users who keep flagging their questions for deletion, over and over again, with seemingly no response from a moderator whatsoever despite the fact that we have responded by declining every one of their flags, with at least one custom message.
Users who don't seem to understand the concept of "moderation", what a moderator does, or when to use flags, and go on to flag things like
urgent plz halp
or
Please answer.
or
I need to use the SQL aggregate function to get all the data from the database. Waiting for your valuable respond...
or
java, jsf, spring
or
it needs moderator ♦ attention
or
Something not quite right? Let us know about it, and please provide relevant links if possible.
or
Something not quite right? Let us know about it, and please provide me with the solution.
A large number of users with these flagging habits flag infrequently. And for as long as they continue to do so, they will never realize we've been responding to their flags. So it's not that they've chosen to persist in spite of our responses, it's that they haven't heard back at all and are poking us because they think they've been ignored. (Sure, you might think it's rude for someone to keep pestering you after you've ignored their first few attempts, but unless you even make your opinion known to them that's not going to make them stop.)
Does this constitute a "serious behavioral problem" to warrant a moderator message or a lengthy suspension (in lieu of a flag ban), along with a permanent mark on their profile? Otherwise, what is a moderator expected to do? Continue allowing these users' bad flags to get in the way of legitimate and potentially severe issues knowing they're just going to keep flagging? They're not a big deal, right, just decline and move on... even if I end up having to do this 10 times a day when I could be handling 20 legitimate flags instead, right?
If I were to propose a feature, I'd suggest having the warning appear to users with, say, a lifetime flag decline rate between 80% and 100%, without the 7-day window.