Users creating and using free accounts have costs associated with them. Those costs need to be met in order for the service to continue, either by tying a revenue model to the users directly, via charity, or by subsidizing their use of your resources by using something they contribute, such as content.

Selling user content is not a stable net in proportion to user use of the site; you don't know that the content will continue to sell at a rate keeping pace with user-incurred costs.

Charity is also not a stable net in proportion to user use of the site; nothing directly and reliably ties charity income to user-incurred cost, and nothing can--it's charity.

The only way to meet proportionate cost incurred by user use is by tying a proportionate revenue to user use. Even if you don't charge a user to use the service, even if you only ask that they fill out a text box to use the site, just as they might with reCAPTCHa, it suddenly becomes 'milking the user'.

Sadly, users no longer merely expect services to be free for them, they get offended if the service provider derives any money at all for their activity. (I think I sort of understand the mentality--"Why are they getting money for my work, when I'm not?"--but that logic doesn't hold up if no money was changing hands while they did the work anyway.)

You're also rather vague about what "the actual problem of spam" is. What strikes you as a mere symptom, and what strikes you as a cause? Yes, these peoples' particular implementation of challenge-response is pretty poor. Is that what you were specifically referring to, or did you have something else in mind?