For me the answer is that they aren't. I know they have a loyal following and that a lot of riders love them, but they just don't work out well for me. I'd rule one out based on comfort alone, but their weight would probably be enough for me to rule them out if I hadn't already ruled them out.

I have owned only one B17 and it was the only saddle that I actually grew to hate. I have successfully used a lot of saddles and generally found that I am fine with the saddles that came with my bikes even for high mileage multi-month touring. I'd happily do another Trans America length tour on any of them.

For me the B-17 was just OK right out of the box, nothing special, but OK. By the time it had really broken in I found it decidedly uncomfortable. I find the Prologo Kappa and some of the WTB saddles especially comfortable, but truth be told I am not that picky about saddles. The Brooks was the only one I have used that I wouldn't take on a long tour.

Note: These comments do not include some saddles that I used in the 50's and 60's. Some of those were awful. They also do not include a couple terrible "comfort" saddles that I briefly rode on rental bikes.