It's the usual comparative-advantage thing, though. Five seconds of your brain reading something is essentially worthless to you (unless someone is paying you to read this comment), but of value to Google. The resource behind the CAPTCHA is of minimal value to Google (what are they going to do with an account at startup-of-the-month), but of more value to you. The transaction is mutually beneficial.
The value Google gets from the thing they want, and the value you get from the thing you want, aren't particularly comparable.
The alternative would be you're doing different work (i.e. more intellectually demanding) on each different site as they invent their own CAPTCHA, and rather than benefitting anybody, it would just be an annoyance between you and the content you want to reach.
IMO, this is strictly better.
Anyway, I am not completely against it, as I said above depending on the situation I make use of it.