Grizzly bears is only a bad card because there are strictly better cards you can get for the same cost. If there was no other 2/2 creature for 2 mana, grizzly bears would not only be playable, but it would be good. Vanilla cards are not necessarily weaker. Any card is weak if there are other cards to compare it to that are far superior. So saying vanilla cards are intrinsically weak is just not true.
Centaur Courser, Elvish Warrior, Blade of the Sixth Pride, Elite Vanguard, Grizzled Leotau, Horned Turtle, Kraken Hatchling, Leatherback Baloth, and many others are all examples of vanilla cards that are decent because they provide a nice power to cost ratio.
So the we aren't talking about making powerful VS weak cards, though printing cards with varying power is a good idea. We are talking about the existence or lack thereof of simple cards, and I am arguing that simple cards are a good thing to have in a well designed CCG. While they may not themselves be interesting, they make the overall gameplay more fun.