If implemented properly it would be a good idea. However such system would not have much use except for strategus battles. That is because if we consider realism here, to be so tired to actually reduce your combat effectivness in some way, you'd need to fight for around 5 minutes non-stop. Sometimes a round doesn't even last that long. So most of the time nobody would be affected by this anyways.
Besides creating a proper stamina system would probably be quite a difficult task, requiring a lot of coding.