If you are lucky, a direct radiator loudspeaker may give you 1-2 % efficiency. To say by using an open baffle we merely trade off a 12db (closed box) or 24db (Vented Box) rolloff for a 6 db (Open baffle) rolloff without mentioning the rolloff frequency had increased from 50 hz to 250 is either intellectually dishonest or worse, ignorant. ...There are NOT "many" (if any) good open baffle designs that go low enough to be able to claim "Full range" status, and there is far too much going on in the low end that adds to the musical experience to simply throw it away for the sake of being different so as to distinguish oneself in a crowded marketplace. Intelligent people can disagree.
Those are all good points, but, as you mention, engineering is about tradeoffs. The 6 dB/octave roll off starts higher, and the 12 dB/octave first-order roll off of the speaker resonance is still there as well, but most unbaffled subwoofers have a low resonance in the 20's so that's not much of an issue.
At any rate, you're correct in saying that dipoles aren't often full-range. Linkwitz's Orion is down 6 dB at 20 Hz, but because of the dipole equalization it can only provide serious, clean output down to about 50 Hz. That said, though, there are advantages to dipole bass such as reduced coupling to room modes (
http://www.aes.org/e-lib/browse.cfm?elib=7579), and, in the midrange, the back and front wave interact in such a way that the speaker doesn't become as directional at high frequencies as it would in a conventional box speaker giving it a more uniform polar response. Additionally, the null in a dipole's figure eight pattern can be pointed at the side wall reflection point to reduce the first reflection.
Anyway, the issues with bass output discussed above are true of all dipole sources whether they're open baffle or not, so if you're going to build a dipole front speaker than an open baffle is as good a way as any to do it. However, these designs should always be active because providing dipole equalization in a passive design would kill the efficiency of the speaker by lowering everything to the efficiency of the bass drivers - possibly the reason for using 3 of them here.