I think there's at least a slight difference between UX design by committee and creative design (for want of a better term) by committee.
UX design in groups makes more sense- there are a series of rules and guidelines you can follow to make a good UI. Everyone can co-operate on that, in the sense that they have a rule sheet, and can assess how the design fits those rules.
The more creative side of it (logos, colour schemes, etc) is far more subjective, so attempting to do it by committee always brings everything to an "offend no-one" median.
UX design in groups makes more sense- there are a series of rules and guidelines you can follow to make a good UI. Everyone can co-operate on that, in the sense that they have a rule sheet, and can assess how the design fits those rules.
The more creative side of it (logos, colour schemes, etc) is far more subjective, so attempting to do it by committee always brings everything to an "offend no-one" median.