Also it's sometimes used as porn-bait (i.e. enter captcha to enter the "free" site).
Human solving is pretty cheap too. I've heard quotes about $2 for 1000 captchas. I guess with 1 click instead of 5-7 letters + enter - it's going to be cents per thousand soon enough, cause you don't even need to know keyboard well or type fast enough. So, I'd say that might be a step backwards.
Also I personally did stumble to Google's captcha in quite a few sites (borderline porn) posing it as their own for registration purposes (sometimes monochromed though). Google has easily recognizable captcha.
A few years back (probably in 2006 or 2007) I talked to a bunch of researches who were working on image recongition and they told me they searched high and low for any proof of this and didn't find it. So I have had a cached opinion since then until about 5 minutes ago. :-)
Yes, however spammers are known to operate on very tight margins (they make a few pennies per million users or something) so even slight additional operating costs can seriously affect their bottom line, possibly sending a profitable operation into the red.
So eventually forcing spammers to employ humans instead of bots is not for naught.
It depends on what type of spam we're talking about.
The obvious here is social network spam (you don't enter captchas to send emails), which is where captchas are most used against spammers, can be very profitable ($xxx per x,xxx-xx,xxx users).
