"I hope that you did not intend to imply that group classes teach bad technique."
While it wasn't my comment, I certainly would make the observation that this is often the case. Unfortunately, in many settings, the group classes are unwisely assigned to junior, technically incompetent staff members. If the teacher can neither demonstrate nor explain correctly, there is little hope of the students getting it right.
In other, more wisely run settings, group classes are taught by some of the most expert teachers.
"I shouldn't have to tell you that teaching technique is not the purpose of a class."
There certainly are classes where that is the explicit purpose, but providing a sound reference should always be important, even when it's not the primary goal. There is just no excuse for flawed demonstrations.
"Newcomers should begin with classes - and not private lessons - because classes are an inexpensive entry point into ballroom."
Provided that the classes provide entry into core concerns of ballroom, yes. But if the only coverage of topics of any real importance is in private lessons, then unfortunately those are going to be necessary almost from the start, if students are to have a chance to become aware of what ballroom dancing has to offer. I would much rather they be able to gain that exposure in group classes, economically, and with a peer group they can practice with - but that just isn't offered in many places.
"Technique may be important to those of us who take dance seriously but not everyone does so. I know many social dancers who are happy simply to be on the floor with their friends."
That's fine as a personal choice. But no one should be making that decision for anyone else by consigning them to a training situation that lacks more accurate information.