What an interesting question!
I can only talk from personal experience, but the main reasons why I bought a Buell (XB9R)were (and remember that I bought it without ever ahving seen one in real life, let alone having ridden one):
1) The way it looked - small, lightweight, nimble...
2) It wasn't mainstream. I know enough people with their R1s, GSXRs etc and its all a big highest up the wall pissing contest. I wanted a bike that would be fun to ride without having to "compete". I was all set to buy a Duke but (oh, the irony now!) the salesman was a to$$er and looked down on me as a lower form of life because I wasn't upgrading my Duke but trying to buy into the "club" for the first time. I thought about a MV but was concerned about the possible reliability issues (

). I nearly bought a Falco - perhaps, objectively, I should have done. But then I got taken by the look of the 9R.
3) I do enjoy fettling and actually wanted a bike that would need a bit of regular TLC. The "ride and ignore from one service to the next" wasn't what I was looking for. At the time I had two other bikes, so it was a fair bet that at least one of them would be on the road on any given day. (Now I'm down to just the XB and its been up on a paddock stand for the last fortnight.)
I could actually do without some of the Buell issues - eg shearing sidestand bolts are just a complete pain. Its a lousy design that probably wouldn't have got through Honda's current quality control (yes, I am old enough to remember the disaster area that was the original Honda VF 750!).
I like the way it tries to shake itself to pieces on tickover (mine does actually tickover now!). I don't mind the regular fettling; although it would be nice if the torque settings in the workshop manual were correct. What gets me is not so much the fundamental flaws but that the MoCo won't address them. I was a beta tester for the XB seven years ago and they are still making this year's models with tensioners that don't work, gearboxes with a "destruct by" date, reflectors on the forks, front discs that warp etc etc.
My next bike? I'm just waiting for the half faired Moto Morini Corsaro. (I'll be keeping the Buell though!)
Dave