I've never felt it's the fact that some people can draw better with a pencil (ok,maybe they can) but I think the doodling is to practice the motion - getting the hand and eye coordination working.
If you're doodling (with a pencil) teardrop shapes, for instance, what do you do - or how do you - extricate yourself from a dead end?
How do you move to another section of the block?
How do you adjust the shapes if you're in a tight smaller area trying to find your way out?
I think it's more to become familiar with those kinds of situations so you don't panic and tense up and/or freeze when you're actually FMQ-ing and confront the same situations.
So like Sadiemae suggested upthread, just keep drawing.