[Jim S]

Mike, how often on a try do you see the defense change personel depending on what personel the offense puts on the field? The purpose of the rule is to keep the offense from loading personel onto the field thereby confusing the defense and preventing them from getting the players on that they might want.
In the case of a try we have the teams at the far ed of the field and the probablility that the defense wouldn't even try to change.
No harm, no foul in this case.
We use discretion in calling all sorts of 'real' fouls. This should be another one.