Thinset and grout are not the same. They are formulated only for their specific purpose. If you had a few small joints to fill, such as in a small repair, you could use the thinset as grout.
On "very narrow joints" as you describe, you shouldn't use sanded material, as thinset is. The grout also has properties that make it set-up quickly, so it can be worked to it's finished look. Thinset is designed to do the opposite- stay fluid for a long time, thereby washing-out the joint as you sponge it.
Spend a few bucks and get the correct grout and save yourself some work. Do it right.
thetiler