I have used my paddocks to round pen horses, often I find when I need to reestablish my need for space and that it's better to be with me than off fluffing around, that I need to do it right then and there, not 5 minutes later when I've marched them down to the round pen.
Though I typically don't use the 'make them work' mentality, I do use it when I feel it's the clearest way to make the point.
Most recently my mare threw a hissy fit when her pony went out of her sight in his paddock. This is an issue she's worked out and doesn't usually bother her any more, but that day it did and she was being silly about it. I went to groom her and she was too focused on where the pony was to stay still and just be groomed. So I decided letting her see that if she wants to move, she can keep moving - that the more she focuses on the pony the more she has to work, until she was content with just my company for the time.
This time went better than usual, rather than me having to keep her moving through all the corners or chasing her up and down the rectangular paddock, she stayed circling me, which was really great! But sure enough, she fell down, just got tangled up in her feet and fell over onto her side >.< My massive draft horse with all four legs in the air, too her a good minute to get up again, she was horrified. I had no idea what to do, to keep going or run over and hug and kiss her precious face. But before I could decide she was off again, so I waited until she submit (she was not limping or anything). It only took less than a minute more before she came in to me and followed me politely as usual.
I checked her over and her side was sore, but not awful. The next day she was stiff, the next she was fine.
I don't think I'll be doing that any more. But this is my personal choice.
I still think it's a great tool, I'll just keep things softer with my horses.