Why do Harbeth speakers have such a cult following

Cult type products tend to appeal to a small segment of the population who see them as special in some way.

I like that definition of "cult". At least as it pertains to "things", stereo eqipment, motorcycles, etc.

They look and sound a bit old fashion, some seem to like that.

"They look and sound a bit old fashion..."

In the sense that a real violin or piano sounds "old fashion" (sic), I would have to agree!

-Bob

11-13-12: Philjolet
maybe the loose cabinet joints that assists in recreating the sound of live musical instrument adds to the 'mystique'

There is a lot of misinformation about the rationale behind the Harbeth cabinet design.
The BBC researched damped thin wall cabinet design was originally evolved to push cabinet colourations down below the critical midrange where they are most damaging to the tonal properties of the reproduced music.
ie, the goal was/is to make music and speech sound tonally like itself, not to create some specific sound.
Those who like this type of tonal accuracy will be attracted to Harbeth speakers and others like them.

IMO, the longer you listen to speakers with this basic tonal accuracy, the more appealing it becomes (and the less acceptable other, less tonally accurate, speakers become). YMMV.