I think the speedometer thing is invaluable. In racine I was POSITIVE I was going dead slow, I think I was surfing a bit and everything was just way too calm, but I was crankin' along at 12.5 kts downwind. Instead of changing something and possibly screwing up what was going down, I kept on truckin'.

Shift indicator, not so much except on long tacks. I don't get much water where I don't have something to reference to. Open water with a really long course it might be pretty handy. If you're slowly getting knocked, it can be easy to miss how far you've gone off track.