According to mates with the rack: makes the bike much more stable when folded/wheeling, protects mudguard, very occasionally used for luggage... If it's going to be a utility bike, the weight penalty is probably worth it, but if you're going for the Ti whippet build, then not.
Yep, on the first day out on my Brompton was welcomed to the club by a mate on his (possibly) titanium frame and suggested the rack on the basis that it makes the bike more stable when folded and much easier to wheel around.
I'm still considering it, it's not much, just £85 I think?
What do people use the rack for? Is it mainly to wheel the bike?