I'm curious as to why you might think Venusaur is BL, when everything you just stated can be done just as well by Meganium.
aside from Sleep Powder (which I mentioned in a kind of hard-to-notice way), I'd say that Grass/Poison is just a far better typing. With the number of powerful flying/levitate pokemon around, resistance to ground just does not bring as much to the team as resistance to fighting (avoid death by CC). Venusaur is a near 100% counter to all the fighting types of UU, including Toxicroak (who I have seen running poison jab more and more often recently), in addition to fulfilling its roles as a grass type. Sludge Bomb also provides reliable and powerful STAB and with 100 base sp.ATK, I can also rely on Venu to put out some real damage, making it useful on an offensive or defensive team.
Picking up Toxic Spikes is useful too. Normally I'd say Toxic Spikes in UU is not that good, but I have seen a few enemy Drapion or Venomoth try to pull it off. While there are certainly a lot of poison types to use, I'd say Venusaur is the easiest general switch in to pick these off the field. Though I personally still wouldn't try to set them up.
Perhaps though what puts Venusaur over say Vileplume though, would be its speed. Even without touching speed at all, Venusaur can outrun the majority of opposing walls. Getting in the first Sleep Powder, or even leech seed/substitute against slow walls with their own set ups is critical.
In summation:
Over Meganium:
+Poison Typing (read: Fight Resist + no poison weak)
+STAB Sludge Bomb
+100 Sp.ATK, meaning Venu has decent sweeping capacity
+Picks up Toxic Spikes
+Sleep Powder
-Some Physical Bulk
-No Aromatherapy (probably wouldn't find room for it on venu anyway)
-No Reflect (I wouldn't find room for it on Venu anyway)
Over Vileplume
++SPEED
+Overall Better Defenses
+Light Screen (though again, whether you'd find room . . .)
-Aromatherapy
Anyway, these are just things I see as being really great about venusaur. In retrospect, whether out-classing meg/vileplume makes it BL or not, I'm not sure and as you said they are very comparable. I don't really want to say, because it's not really obvious whether or not he over powers other pokemon.
Let me say it like this: As long as Venusaur is in the tier, there is no reason why I would not run it on just about any team I would make. I'd say the same for Garchomp in OU though, and it's free to argue him either way as well.
The others I would not say this about. Aerodactyle has its own specific uses, and does not outclass Swellow, hitting a lot weaker and having no priority moves (though better speed and coverage). Weezing obviously cannot function very well on an offensive team (which I'm partial to), and anyone would have their concerns about running a wall 4x weak to rock (articuno). I've seen like 2 articunos this whole month, tops.
edit: edited the part about the screens