I agree with NeoThermic that this kind of solution (Q&A, with or without images) would need some more time to set up; I don't agree that it would take so much more time, and I also don't think this should be a stopper, if the solution is effective: do you want a better protected board? then you'll have to take the time to set up a better protection environment. For many admins of medium to big sized boards this shouldn't be a problem. The others... well, they can still use captchas or whatever. I mean, it's not an aut-aut decision, in theory you could implement both (or more) measures and then give the admin the option to choose which one(s) use and configure it/them appropriately.
As Drew said, I agree both things have their pros and cons. I don't think Q&A is the solution, because, once it is standard, it would have basically the same problems of captchas: being easy enough to break, or being difficult enough to keep some people out.
pick a word, go to google images, select a suitable image and up load it as you would an avatar, enter the word in the expected response field and you are done.
Here I have to say you've just provided a hint to spamming systems: try registering, you're shown an image; now just google, find the same image and use the corresponding words.
So it would still be a little more complex than this.
So, to sum it up, every system has its pros and cons, and every system has a strong con when it's standard in hundred of thousands / millions boards.
Maybe a way could be to develop and implement more than one system, constantly create new ones, and let admins easily find and install new modules of this kind. Of course lot of work here; the kind of work that needs a community-run development more than a core-developers-run development. (After all these are, or could be, quite stand-alone modules).
Last two things: I also suggested to split the topic , and Drew please avoid quoting entire messages, it makes the page really really long.
/end of my 2 cents.