You are however denying yourself the skills of the outfield player who is going in goal who was presumably deemed to be of greater importance than the player coming on or would not have been selected in the first place.
There's a hell of a lot wrong with the club at the moment, but this I'm not too concerned over. Its a very rare occasion it could happen, more chance of a outfield player getting injured so you could argue what if both your centre halfs get injured but you've only got 1 on the bench cos you put a keeper on there.