A PCM data disc (.wav files burnt to a data disc) won't play on anything. Well, I suppose it'd play on a computer... But not an audio CD player.
Lexicon / Nomenclature Alert: It's not a glass master - The glass master is made in a clean room by guys in space-suits. The master disc is generally referred to as the production master or replication master. [/Lexicon Alert]
Anyway -
If you're planning on getting CD's made, you don't have much of a choice... If you're simply using digital distribution, then it's not really an issue (although I'd personally go with at least one replication master "as it's supposed to be" in case of any chance of having discs made at some point).
In any case, I can think of several reasons to have a replication master and no reasons not to have one.
That said - Probably a third of projects that come through here are two-way FTP delivery and the client is either using digital distro only (iTunes and the like) or they're "burning their own."
THAT said - I typically do a little 'snooping' to make sure they understand and have the ability to actually author a compliant disc.