Even if you need to sub your goalkeeper just once in a season, a potential 3 points lost can be the difference between 2nd and 1st, promotion and the play-offs, relegation and survival etc. I've never felt like I needed another on the bench anyway.
I never put a keeper on the bench. I feel like like I could always use that extra choice of a sub, not to mention I don't like keeping my players off the bench (they seem to get ******). I always have at least 18 guys that deserve to be part of the match squad. I've had a keeper get injured maybe only once or twice ever in playing FM.