If the money then circulates then that's OK. If the money is withdrawn then take a good cut of that.
One downside to that is that the more popular it becomes, the more 'currency' the flattr-coin becomes, it simply circulates with people neither depositing it nor withdrawing it. However, flattr are then effectively become a central bank and that isn't a problem.
The real problem with not taking a cut of moving between flattr accounts is that people would sell flattr through side channels to avoid flattr taking a cut. So you'd pay me cash and I'd transfer to you the flattr coin on platform.
And if it took off in a big way then money laundering becomes a problem and the cut would not cover the cost of KYC etc.