I wear a tallit katan at almost all times and I do, in fact, sleep in it. I spoke to a rabbi about whether I would actually ever make "al mitzvat tzitzit" and it seemed that the answer was no. Kitzur also informs me that one doesn't make the blessing when putting on the tallit katan after a shower, as far as I know. So, if I'm donning a tallit gadol every morning for Shacharit, will I ever actually make "al mitzvat tzitzit" on a daily basis?
Not on a daily basis, but maybe on a weekly basis if you put a different tallit katan on for Shabbos. Also, summer's coming up and if you go to the beach or the pool you'll almost invariably have your tallit katan off long enough for there to be hesech hadaas, so when you put it back on you'll make a bracha on it.
I think some people might argue that in both these cases you can have intention to cover these situations when you make your bracha in the morning, therefore would not make a bracha, but personally I never have the presence of mind to think that far ahead.