I have an idea in my head of what I want the coaster to do, so I want to see if it will fit or not. Something similar happened to the tight fit contest, I had my idea, built it, but it wouldn't work.
If not, I would be signed up for no reason. Putting more work on yourself, but that's fine with me.
If people were to start early....so what? I guess I don't see the point with people having different lifestyles with work, family, etc. and having more time would be beneficial for a good coaster.