Why would I want S/MIME?

[Robert J. Hansen]

> Assuming everyone is willing and comfortable with using GnuPG, is there any
> compelling reason (aside from easy setup and use) to use S/MIME?

Regulatory compliance.  For instance, if you were in the banking industry you'd be using S/MIME even if everyone preferred GnuPG -- S/MIME is part of several important banking standards, whereas GnuPG isn't.

That's the only compelling reason I can think of.