I actually looked through the equipment section of the UCI rulebook tonight and couldn't find anything about it-- I had thought that the thing with the SRM computers was related to protrusions from the handlebars, but that doesn't seem to be it. It could be that they count as an electronic pace device and you can't put them where you can see them in a TT.
In a mass start they're irrelevant and a distraction. You have to cross the line in front of other people at the appropriate times and how fast you're going when that happens doesn't really matter. A friend of mine used to try to convince me to use one "so you can know how fast you have to go to take a lap". I know exactly how fast I have to go to take a lap-- faster than the pack. And I can see them and have a lot of fixed reference points to see if I'm gaining or losing.