I prefer to let @vit, @kimamila or @Galuel answer you about Geconomicus rules and results, they are the specialists out here 
Yes MetaBrouzouf currency isn’t constraining at all. Still, you have to consider that the certification process will be a long time process, or should I say a human time process.
So certifying someone will be costly for a certifier, because:
- he will have a finite stock of certifications
- he can make only 1 certification a week
These constraints, added to the distance rule, avoid sybil cancer. To sump up, these rules are just technical rules avoiding massive automated cheating.
Still, we shouldn’t forget about people who will organize themselves to create fake identities (at a human time scale though, since massive automated sybill attacks won’t work). We consider uCoin protocol shouldn’t care that much about them, and let the detection and ban tasks to humans directly.
For example, humans may create public websites displaying a list of potential cheaters according to blockchain analysis (should I recall that the blockchain is public, so anyone can check who certified who and when). This analysis could also be linked to a public place where people publish informations about their identity (why not State informations if they agree to do so) to strengthen the possible trust in their unique & living identity.
Maybe this is a mistake to think this way, I don’t know yet. All I try to make is a flying prototype. Maybe this won’t ever fly! But maybe some people will learn about our mistakes and makes something that will actually fly.
Grant me the benefit of doubt 