The issue here is NOT (believe it or not) whether Mence should be tested. I have no problem with a fair test to see if it's broken or not in the OU metagame. The problem comes with implementing the results of the testing.
Now that Latias is gone, Mence has lost a potential counter so it's a greater threat than it was (and it was already pretty grizzly). My fear is that this will be reflected in the results of the test. Pokemon have variable threat levels depending on the metagame. If Mence is taken away, the metagame shifts drastically and a very powerful beast is removed. Something else will step up to the plate and there will be new sets and counter-sets for a while until the next threat too is deemed too powerful for OU.
In actuality, the power a pokemon has is relative to it's current metagame. Banning a mence is a temporary solution to a greater problem, namely: lack of standardisation across pokemon. The need for implemented tiers means that the system is already imperfect, and while the wonderful work done by smogonauts and the like to rectify this is a worthwhile endeavour, it is a treatment, not a cure.
I'm not proposing an alternative, since I don't think one exists, I'm just saying that sooner or later a line will have to be drawn. A powerful pokemon (like Mence) will emerge to dominate the metagame. Now, if it's banned that doesn't solve anything. It just buys time until the next monster comes along that people have a hard time dealing with. Rinse and repeat.
My worry is that the Ubers tier will become populated with 'Neo-Ubers'. Pokemon who have no real business in the Uber's tier but who became OU's biggest problem by means of elimination.
I say leave the threats in OU alone. Sure they're hard to deal with, but possibly not as hard as whatever will appear after you shift the metagame by banning them. For remember, when you ban a pokemon from OU, you ban a potential OU solution to the new threats that will arise.