If you really want to run one then there is nothing wrong with it, it's just not necessary whether its a 10g reef or a 200g reef. In a sumpless tank it could help with water clarity but that's about the only benefit you would get with running it. One thing I would like to add however is that you can run purigen or a phosphate removal media in a HOB
filter whereas you couldn't with a powerhead.
Running a 75g tank, especialy if you want to turn it into a reef I would highly recommend getting at minimum 2 but probably 3 powerheads whether you are running a filter or not to help create a more dynamic flow.