I think the main problem with allowing NFEs into lower tiers is the risk that UU will just develop into a water-downed version of OU. Personally, I turn to UU when I have grown a little stale with OU.
Allowing the likes of Hippopotas means that many of the OU tactics will still be prevalent in UU and the difference between the two metagames will become slight. Weaker Pokemon, same style of play.
Lets say I become a little disillusioned with Hippowdon and Garchomp running riot everywhere, so I make a UU team and go into that environment for a while only to see Hippopotas and Gabite everywhere. Sure, the opponent is disadvantaging himself because Sandslash and Gligar or whatever would be a much more potent combination, but the principle remains. Do we really want UU to be a mini OU? Or do we want it to be a thriving metagame by itself?
In BL, when you're generally fighting slower things with less Special Defence, the difference between Kadabra/Alakazam and Haunter/Gengar is much smaller.
I think the list of permitted NFEs will need to be looked at carefully. Obvious things like Pikachu, Vigoroth, Scyther and Trapinch are cool but other things like Magneton need to be looked at. Sure, he's generally inferior but that little extra speed does give him something to work with, especially with all the 80 basers running around.
However, I am against the likes of Gabite, Shelgon, Dragonair or whatever for the reasons stated above.
So basically, you don't want to laugh out load as your opponent uses a pokemon that isn't a physical wall or threat to set up Sandstorm with a pokemon that isn't a physical sweeper to abuse it.
Give me a reason why anyone would use Gabite over Sandslash. Is the 82 Base speed really worth being clobbered by any attack from any attacker? 68/65/55 is depressing. Sandslash at least has 75/110/55. With more attack (100) and Swords Dance and Night Slash (better than Shadow Claw), Sandslash abuses Sand Veil just as well as Gabite. Gabite's only real advantage would be to run a kind of UU ChainChomp set, but 50 SA is really bad, especially when most of the walls are special, and Chainchomp takes the EVs out of Speed.
The pre-vos of OU threats are just too weak to be effective. Anyone worrying about OU-lite is kidding themselves.
We've had UU for 3 gens. How many Dragonairs, Pupitars, Shelgons, and Metangs have you seen? Gabite will be no different than his predecessors.