[css3-regions] region name - why is it string, not indent?

When we discussed it at F2F, it seemed that using <identifier> type for flow name would not be appropriate, because the ident could be anything and it might conflict with current or future keywords. Is there a rule or an earlier decision on where ident is OK to use and where it has to be a string?

There seems to be a precedent of 'counter-increment' being <identifier> (with the values of 'none', 'inherit' and 'initial' being not allowed). Is that approach now considered inferior?

Would be good to have a reference to reasoning that supports the decision.

Thanks
Alex