Trev if all the live draft spots fill up Yahoo simply opens up more times. I've created leagues before where the only open times were early morning on a wednesday. I just kept checking back each day and eventually they opened more times and I got one that I wanted.
Would you rather have people not do mock draft leagues and simply join some other league where half the people are actually trying to play while the other half stopped checking because they were just getting a practice draft in?
I think it's possibly due to the fact that a league couldn't fill up their league and had to choose another time to draft. I could be wrong but i've noticed the same pattern of checking back and seeing a time available.
to answer your other question.. YES!
i'd rather yahoo offer a "MOCK DRAFT" option so anyone who wanted to practice could do so all they wanted and let those of us who want to draft a real team do so. Either way it won't affect our keeper league THIS year because we're doing an offline draft...