I think it can be very useful for da streetz if you are very good at it. With shoes on, I think a kick of one of those guys to the liver or to the solar plexus is deadly. Now, if you are not very good at it, it's just a recipe to get your kick caught and slammed on concrete lol, that's why I would never kick in a street fight unless I really know what I'm doing.
As far as mma or muay thai goes... I don't see much utility besides the stomp kicks. Their kicks lack stoping power, many are completely inapplicable (for instance they often throw tripple head kicks, with the knee being up all the time) so that I feel even the flashy TKD kicks would be better to learn, at least you can legitimately ktfo someone with those... Last but not least, getting used to kick with the foot isn't doing any good for you if you want to fight barefoot...
I think a good pressure kickboxer and good leg kicks (like a Joe Valtellini) would just walk through their kicks... They also don't seem to sit enough on their punches (nor do they on they kicks).