i can see that system failing miserably.
i was once !#@$%ed out in a raid because i "already had a better offhand" after i won the roll, when they were going solely by ilvl and not paying attention to the fact that i was WAY over hitcap and no longer needed the hit on my current offhand.
if blizzard were to implement some kind of "you already have better" system, it would almost assuredly go off of ilvl, and if you think gearscore is bad... that would only be the beginning.