Hey Rok
This is something I'd really like to see more of. I've recently become a witness @steemcommunity and our focus (as the name suggests!) is to support communities by way of helping retention of existing users.
As @paulag highlighted recently in the link below, we suffer hugely from retention issues, but with a 'simple' League of engagement that I've been running for 6 months or so now, the retention rate of these users, joined since Jan 2018, is up at around 85%, as apposed to the average across the platform which is more like 40%.
As I said, we are new to the game witness-wise, but have our policies laid out clear, and are not into the bidbot craze, or exchanging of votes to rise in the ranks.
The stake delegations to communities that are then sharing this out to the keen and engaged sounds like a solid use of centralized SP - this is what we do individually, and will continue to do as time passes.
If you have a little time to spare to check out our mission, i'd be very grateful, cheers.