I was just wondering about the blogs users create on my Multisite install using Buddypress and Pro-site plugin.
If these contain low level content or partly finished blogs... will my main site get penalised ??
Thank you for the great question and to give an honest answer only the people that write the algorithm for the search engine will indeed know this answer for sure - and they are not going to tell us.
Will they punish the main site form what I know about marketing and search engines, probably not.
You did mention Buddy Press and the fact that it can only reside on one site in your network because it is not MultiSite compatible if that is on the main site certainly not.