That recaptcha thing is a supported whatsit. According to the site it just needs four lines of code to link the site to their test.
If you still get lost of bots running through the latest captcha and I had to look twice at that to get that right, it might be worth trying.
The line through words is apparently one of the best ways to fool bots, (from more than one source) and having an outside site provide the data might be a good way to utilise a larger database.
You are right though, bots will get through almost anything and if the bots can't they farm it out to human operators anyway. Probably never win...