The patch was initially from Serhiy as part of issue23591. So it's safe to say at least one person requested it, and a core dev at that.
I will happily make it two people: as an occasional user of re having the constants be named makes it much easier for me to use; I daresay that is true for other occasional users. IIRC giving names to numbers was one of the motivating factors in having Enum in the first place.
I do agree that RegexFlag is a better name -- I wasn't real happy with Flag but didn't want to miss the cutoff.