Thanks for that detail!
Your use case of “have client sign up for (free) account, and invite you to collaborate” is totally fine with us! Login sharing is not fine. Then would of course also pay $15 for you to collaborate since collaborators on the free plan do cost $15/mo. But then it would all be on their credit card, and if their site becomes popular or builds a lot, they’d pay the cost.
It is our intention that if you resell your netlify hosted sites (e.g. “I made this for a client who pays me”), you also wouldn’t host for free - so this will achieve that point as well. If you do resell for one or more clients, you really should be paying anyway, that’s our intention as well.
Hosting your sites for free is a-ok! Charging someone else to host their sites and not paying us, not so a-ok 
Thanks for the question!