I think mostly everyone who rides Minis also rides big bikes on the track as well. They both teach valuable lessons, but I can assure you that I became a much more proficient rider on my big bike as a result of the things I've learned on the OMRL bike.
It's cheap, they crash well so you can learn your limits and they don't have pristine brakes or suspension... so you learn how a bike feels in different situations without much risk of doing tons of expensive damage to a bike or to yourself.
Are big bike track days fun? Absolutely... but they're far more expensive and thus... less time to do those types of things.
My only argument is if you HAVEN'T tried mini-bike racing, you really can't talk about it one way or the other. And you mostly don't have an excuse to not try it as there are plenty of guys on the boards here that are willing to lend their bike for ABSOLUTELY FREE to come try it out on a practice day.
I dunno... I just don't see why someone wouldn't want to at least try it....
In the end, everyone has their own agenda of fun...
Mine just is currently racing mini-bikes for $70 for an entire weekend for 2 days of riding and fun having.