The rack if you're going to tour and carry all your gear, otherwise no. I bought a rackless version. Do get the luggage block though. I got the reduced gearing 6 speed. That was a WISE choice.
Like I said earlier, I looked at cheaper bikes which also rode well but the Brompton is special. It's not twitchy or any of the occasional complaints I've read from others, at least not for me. My initial test ride was in El Segundo, CA which is a hilly town and I took to it immediately.
Was the extra cost of the Bromptone worth it over the many years I'll have it? No question. And the fold and folded size was a major issue for me as I always want to have it in my rig.