Ed, my guess (and I have MUCh less experience than you do) is that BLH horns are built if someone is not using a subwoofer. If one is using a sub and crossing the "full range" driver at 90 Hz or so, then I have not seen anyone use a horn.
Now, with a real low Q driver that is not really meant to produce a lot of bass, like a 3-4 inch fostex, one may need a horn to get down to even 80 -90 Hz, in which case it is totally justified. But if one is using an 8 inch full ranger, that can achieve 90 Hz in a simple BR all day long, the reason for a horn may become less obvious.

What do you think?
-akhilesh