I have a key example of motivation driving software acceptance: My mother and my mother-in-law on Facebook.
Neither of them is particularly tech-savvy, but once they heard there were adorable grandbabies on Facebook, they were logged on right quick.
The motivation factor is a key problem at my school. We spend a lot of time teaching students how to do basic tech tasks because they were never motivated to learn them. They can download illegal music on their phones, but they never learned how to attach a file to an email.
In his video, Prof. Sherman talks about making the signup process frictionless to get users into the product before their motivation wanes. In my opinion, Pinterest may have taken this a bit too far.
As I signed up, it required me to select five areas of interest and instantly presented me with a premade board. But I had to hunt a bit to find out how to make new boards and even how to create my own pins. I suspect this is driven by business requirements rather than straight usability. I believe the company is trying to make things easier for casual users, in an attempt to gain more market share, at the cost of making things a bit more tricky for power users.
Extrinsic motivation can be a crutch for usability, as Hootsuite founder Ryan Holmes found. He found he got better feedback from free users than enterprise users, because he could not count on their loyalty. He had to work harder to keep them, and it made the product better.