By the same logic, you should make high level items stop rolling low level mods.
With less crap mods cluttering up the mod pool, you've got a much higher chance of rolling the rare you want, it's genius!
Sorry, but I disagree. Somebody who wants a specific low level unique already has the option of farming a low level area to maximize their chances, and finding bad items is a part of the overall experience imo.
Additionally, I imagine that rebalancing the rarity of uniques with a maximum drop level would be very time consuming.