Subbing someone in practice isn't that big of a deal. A lot of times, the better fighters put themselves into tough situations to try and work out of it. They can't always get out of it but in a real fight they never would have been there in the first place.
Also, you could be the 10th fresh person in a row to roll with one of the fighters who is building up for a fight. Subbing him when he's tired and you're fresh is very possible but doesn't mean you're a stud.