I'm not your agent, or your parent, and I don't think these sites are a total waste of time. So please read my other post and/or contact your agent before making a profile. Mostly speaking about Model Mayhem here...
So I just posted something about how these sites can waste time, and they can, but, they can be a great resource if used in the right ways, which is networking!
Where else can you find other people in other cities/areas/countries? The site is a huge melting pot of everyone from around the world. This can provide a great way to network with others, make new friends/contacts, and find places to crash when traveling. Granted there needs to be some common sense used when talking with internet strangers, but I've made some great contacts over the years and it's a great way to get to know people you otherwise may not have.
So, and again it'd shy away from shooting with just anyone, but a simple profile with a few simple shots and snapshots (front & side in a swimsuit and headshot) may help network with others in different cities you may want to move, photographers in other areas, or just a good waste of time in the forums "discussing" stuff.
Again, I'm not your agent, parent, or a doctor, but there are good points to these sites and this is a big one!