IMHO, WGT should limit it as follows:
Rookie: Tour Masters and below only
Veteran: Tour Legends and below only
Expert: Open to all tiers
the problem with this is that the labels are nothing more than WGT's use of terms that just differentiate the entry fee but really have no real relevance to the actual skill level of those entering each division.
Rookie will actually play more challenging than Veteran and Expert - especially when you factor in higher tier players are going to increase their chances of being matched up against a low tier player that is more likely to rage quit on them - but it is something they will risk for 10 credits but not 100 or 1000 credits.
my only complaint is when I see a tour champ or any player possibly consistently dominating the Rookie tier - winning over and over again - but never moving up. they are doing this while playing 1000 credit balls - they don't even care about the cheaper balls they would win because they are not balls, they are interested in.
My suggestion would be - WGT should impose something like a tier-based promotion system so that if someone wins a lower division X # of times then they are no longer eligible to play in the lower division.- they will have earned their promotion in essence. just as they promote some coin room players based on their success in coin rooms without having ever reached the ranked round requirement.
That way, tour champs that are still building their apparel and experience in coin rooms are not unfairly penalized and forced to play in higher divisions they would not be competitive in because they may even get beat by lower tier coin room specialist - but the players dominating - would not be permitted to continue doing so.
Given the range of skill levels at every tier - only forcibly promoting those dominating certain divisions should be forced to move up - otherwise players should play where they feel comfortable because not all tour champs play like tour champs.