Another new story, but only a partial

I'm off and writing a new partial for my agent. Even though I've written one other middle grade partial, my agent told me to start on a new story while she's shopping the one full MG and the partial MG manuscripts. I guess from now on, since I've written a full in each genre I write, I only need to write partials for her to submit. I'm not sure if all agents work this way, but mine does. And, I have to say that I really like it. I don't have to commit myself (or the time) into completing an entire story. I can write a partial and synopsis and then move on.

The only downside that I see to this arrangement is that perhaps my enthusiasm for the story may wane if I don't keep plugging along. I've got several women's fiction manuscripts that are partially completed because I lost interest. But those cases are different, I think (I hope) because I hadn't written a synopsis and they were written early in my writing career - meaning, I've learned a lot more since then and I'm sure if I went back to those, I'd see the problem I lost interest - no clear story problem.

Either way, I think it's pretty cool that I only have to commit to three-five chapters and write the dreaded synopsis.

How do you write? One full story at a time, or in partials?