We all have different motivations and respond in different ways to different stimuli. Some people are motivated to do something by the inherent worth of doing that particular thing (like the joy His Little One gets from the simple act of serving her master) but others don't. (In my opinion, that doesn't work for most people.) Otherwise noone would need a paycheck to go to work and our capitalist system would fall apart.

I don't believe it is wrong to motivate people with rewards--it works for kids in classrooms, dogs at obedience schools, and executives in boardrooms.

As far as whether it is legit in the context of your DS relationship, that depends on *your* DS relationship and not some arbitrary ideal of service.

Frankly, I don't sub to serve. I've served lots of people all my life in a hundred different capacities and I'm a nice helpful person, but I sub sexually because it is intensely erotic for me and my partner(s).

Ultimately, if it works for you and it works for her, helps her with her issues and also makes you happy--what's the harm?