True 2D games COULD be made using 3D graphics...but you lose detail and personality.
The only thing developers gain from making a 2D game with 3D graphics, is the ability to use some graphics tricks for special effects, and a cheaper/easier animation and character design process. I guess that can go towards easier level design as well...but really good programs can do that with 2D level design its just harder.
What do you lose? All the character and personality and charm of hand drawn sprites.
Best Example:
New Super Mario Brothers Vs. Super Princess Peach.
Based on graphics alone which would you rather play?
Based on graphics alone which has more detailed levels and characters?
Based on graphics alone which has more personality?
I think most will agree that Super Princess Peach definately wins the contest, whether you want to play the game more than the new Super Mario is a matter of opinion.
But you will say graphics have no barring on gameplay, so you shouldn't judge.
Actually graphics do have barring on game play. Take Super Mario Brothers, and then the Super Mario Brothers from SUPER Mario Allstars. Which would you rather play? Sure some would argue the classic, but if you weren't judging the game on classic status you would choose the newer one.
Also imagine Super Mario World 2 with 3D polygon graphics and the whole 2D stetch look gone. You won't get the same feel and satisfaction of the butt pound kicking up dust, or the combating the strange creatures in the make up. The graphics were the building blocks for the entire feel of the game.
2D games should remain sprite based when at all possible. I know this is becoming less and less likely, but until 3D graphics can capture the depth of character design sprites can I don't want to lose my 2D sprite based games.