The overhead of managing two RAID arrays would impact performance, probably somewhat significantly, which would be another argument against this.
I'm not sure why you take such offense, I think we've presented pretty good arguments why this is not really a desirable thing to do. Hard to imagine you think that creating two or more RAID volumes would be less work than simply setting the quotas for the users. In addition, when a new user comes along, you're stuck, because you've already built your arrays, so you have to start all over!