Apple is not relying on the public to catch serious and obvious bugs. They have plenty of staff on payroll to do that along with automated testing. However, public beta testing gives Apple more opportunity to catch subtle or obscure bugs.
The code doesn't change significantly between beta versions. Obscure bugs on Beta 7 are likely to show up on Beta 10.
Apple needs time to prune references to unannounced devices from the code. This takes time and resources. Giving the public Beta n-2 or n-3 is a good compromise between security and public outreach.
OK, thank you. And do you know how they choose which build will be beta build?