Spinning kicks in Muay Thai?
At my Muay Thai gym we train spinning kicks rather often, at least relative to any other Muay Thai gym I've ever been to. Earlier incarnations of Thai boxing actually did independently develop some spinning kicks such as the spinning back kick (kwang leaw lang, or "deer turns back") and spinning "hook" kick/reverse roundhouse kick (jarake fad hang, or "crocodile whips its tail").
I'd say certain spinning kicks aren't bullshido. The spinning back kick and spinning hook kick can be used, though rarely. They are lower percentage kicks, but I don't think they're useless enough to be considered bullshido. I've seen them score KOs and crowd-pleasers in a good amount of Thai fights.
I've been in fights with TKD and Karate practitioners, though, and they most definitely overuse spinning kicks. They can be easily countered by 1) simply throwing a cross; 2) executing a low round in mid spin; 3) sweeping the supporting leg while they're throwing the kick; 4) moving in out of (in the case of the hook kick) the way of the heel or stuffing the spinning back kick or side kick and suplexing or off-balancing the kicker (I've seen this employed by many Thai boxers against TKD and karate fighters, works every time). And not to mention the fact that even if the kick does make contact with something and doesn't miss completely, odds are its being blocked normally, and in that case the kicker would loose some balance and could be easily kicked, kneed, elbowed, punched, sweeped, etc. This of course isn't helped by the fact that TKD and Karate (for the most part) are seriously lacking in good defensive technique and footwork.