This isn't actually correct. That is, perhaps you consider 640Kbps DD+ a joke, but the actual spec for DVD DD is maxed at 448Kbps. This means that 640Kbps DD+ is a VERY distinct improvement over virtually any and all DD DVDs. I'm only aware of one DVD with a 640Kbps DD track (Pink Floyd's Pulse Concert) and it includes a second DD track at a standard rate in order to satisfy the DVD Forum spec. Perhaps there's another somewhere, but I've yet to learn of it.
If you've ever listened to a 384Kbps DD soundtrack, then you probably realize 448Kbps sounds significantly better, and yet there isn't really all that much bitrate difference. Well, 640Kbps is a significant step up over 448Kbps.
I agree, I'd go for 1.5Mbps DD+ over 640Kbps DD+ any day of the week, but both are a dandy step forward from DVD DD 448Kbps.