Currently there is no option to limit how much resources each named user can consume, so it would be nice to have some sort of a "quota system" to prevent them from exhausting the servers resources.
I've seen a similar idea for ArcGIS Online here.
Perhaps the solution to that idea could be harnessed to Portal if both the credit and the quota system were implemented in Portal.
The beauty behind the credit system is that all system resources converge to a single coin: credits. Processing, storage and bandwith consumption are all translated to credits consumption.