I think it's an ergonomic thing. I've got a BO 4 thumb which looks, sounds and plays fab but is soooooo different in 'position' to my fenders due to the short top horn. I think if you only play one bass all the time - these are great, but if you have a bit of a stable going, it puts you out of your comfort zone gigging with a thumb. That's my ha'penneth.