Hey Marc! Yes, affiliate programs are awesome for SaaS where the margin is really high. Giving away say 30% of that initial revenue for a new customer is a great trade for the life time value of said customer.

Rewards can be structured in a ton of ways (one time reward, recurring, percentage, flat, etc) and it's good for any business model, not just SaaS.

And you're right, they are almost always cash rewards instead of service rewards like Dropbox does. That is more of referral marketing than affiliate marketing.

Let me know if you or anyone else has more questions!