The point is that all these measure are 'trivial' to break, and so are captcha's. Except with captcha's you impose a burden on your user, and with other techniques you can offload that burden to the developer. I'm not sure what the 'existing' part in 'existing spambot' has to do with it - the time it would take to add farmed captcha solving is marginal (you don't even have to mech turk it - most captcha's are broken with OCR software readily available on the underground market anyway).

captcha = sign of clueless or lazy, or both, developer. I don't put up with it anymore - I have yet to meet a single registration that I actually need that uses a captcha. I'm not the only one, either.