Never saw them in a dairy situation.
One of the lessons of successful people is to look at other successful people and do what they did. Of course there are innovators and those who blaze their own trail etc. but for the most part, if you don't see a bunch of people doing "it" in your area there is probably a reason. Yes Dexter's are a bit smaller and easier to handle but they also dress out proportionately less. You spend the same amount of time on them as you do an Angus. Its sort of the false economy of narrow gauge railroading mounted on hooves. Now I do have a number of neighbors who have succeeded with Simmental cattle, which might offer a similar option for you to consider.