This is a common misconception but it's just not true. As you can see in this video mobility is hardly a problem. It certainly would be possible that a piece could be bent or damaged badly enough to hinder you, but a properly fit set is going to let you do whatever you need to do in a battle. You are certainly right that it was expensive though, full plate was similar to buying a luxury car. It was rare but not that rare, a sign of wealth and social class but not like only kings could wear it. And it certainly was not strictly ornamental.