Part of the problem there is that in some fonts there's very little difference between a "q" and a "9". So that's either "qA726F" of "9A726F". Either way, I didn't find that one very difficult, and if the others were similar, I wouldn't've expected a problem. A side point is, of course, that most CAPTCHAs of this style don't actually need to be that complicated.

That fact that you had a problem does show that this style of CAPTCHA has to be deprecated.

People are reporting that hidden fields with names that suggest they are necessary - and hence get completed by robots - are very effective, as are simple questions in natural language. Point is, currently they're not common, so spammers haven't yet had the motivation to crack them.