Another voice chiming in in favor of keeping private games. The bonus is as much psychological as anything else; knowing that it'll be very difficult to have someone looking over your shoulder is rather an aid, most times. Additionally, the private thing adds something of an atmosphere to things - more homey, the knowledge it's just you and your players, no outsiders possibly rooting about in it. None of the private games I run ever advertised; they all came about after an exchange of PMs.
No footprint, no impact, and just two or three guys mucking about without having to worry about someone peeking, and they never need to know. This is a boon I'd rather not do without.
Of course, if the admins decide to go with removing the feature, I wouldn't protest.