For mine I think keep them out, leave the digital speedos to arcade games. Actually listening to the car is going to tell you more than a speedometer, anyhow.
In a race the last thing that's on my mind is how fast I'm going, unless I'm heading to pit lane, if you're focusing on your speed in a race, it's going to cause accidents.
Sorry if this post comes across as aggressive, I don't mean any malice.
Each to his own, but I think leave them out =]