A macro lens would be interesting for tiny toes, etc., yes. Also nifty for bugs and flowers shot at a distance of just a few inches from the end of the lens.
But if you are planning on only having one good (I would not count the kit lens as "good") lens for any length of time you will find that a fixed length lens at 100mm is too long (too zoomed-in) for a lot of shots. On a crop-body that is, well, 1.6x more true.
If I were going to have 1 single prime as my only lens, I would choose something of more average length, like 35mm or 40mm or 50mm. But still I think most shooters with only 1 lens would opt for a zoom lens, giving you a range of lengths (17-55 etc..), and maybe make a prime their second lens later on. If you are shooting hands, even tiny ones, rather than really tiny stuff like aphids or flies, you can crop a photo taken with a good-quality normal lens enough to get those finger shots just fine.