The problem isn't captchas, but users not understanding how to interact with them. So what if a few are bad? Hammer out best guesses, fast as you can, until you're successful. It's not as if you're graded on accuracy. There is no reason to ever resort to the refresh button,

Out of curiosity, I went and opened the demo page (https://www.google.com/recaptcha/demo/ajax) in a new incognito window and timed myself. I can do about 8/minute at maybe 90% accuracy.

Captchas are only a problem if you compulsively refresh in hopes of getting something clear.