I decided to do all vanillas at the same time since I will pretty much say the same thing about all of them. First off, what is a vanilla? A vanilla is a creature with no abilities whatsoever. The 5 creatures posted here are the archetypical examples of vanilla creatures (notable exception: Centaur Courser). Now, why are vanillas even made? If WotC wanted to make terrible creatures, they could just make more Chimney Imps or Wirewood Elves. That is true but are vanillas really that bad? Look at Grizzly Bears; it's a 2/2 for 2 mana and that's not that bad. Sure, it can't do much except for attack and block, but sometimes that's all you need. There are some vanillas which aren't worth it though. The formula that I use (yes, I sometimes use math in Magic) to determine if a vanilla is worth it is:
Power = Converted Mana Cost (P=CMC)
Toughness > Converted Mana Cost
These formulas have never failed me before and I don't think it ever will.
Vanillas are so engrained in Magic's history that they've become part of our vernacular. Dawntreader Elk, for example, is known as a Bear with an ability. By the same token, Bloodline Keeper is a Hill Giant with an ability and Dauntless Escort is a Centaur with an ability (Cadet/Merfolk aren't in the vernacular yet, but I'm going to make it my mission to see it done).
Pros: They don't have any negative abilities
Cons: They don't have any abilities
Rating: 1/5 (if they fail P=CMC and T>CMC), 2/5 (if they follow P=CMC or T>CMC), 3/5 (if they exceed P=CMC), 4/5 (if they exceed P=CMC and T>CMC)