Herne Hill Saturday Training

I guess because it is another potential distraction to concentration. Clearly most people manage fine with one though, as there are a lot who do it. I ride with one in my pocket, so guess I need to check that is legit. If not, I'll stem mount and tape over the screen so I'm not distracted! Regarding effort, isn't perceived effort enough for the track?

Stoopid to have things in pockets (stoopid to have pockets?). Not only if it falls out and causes a crash, but if you crash and it injures you if you land on it (kidney punch).

Interesting what you say about perceived effort. Yes, a speedo does help with pacing judgement as you can see the effect of the banking and your corresponding effort adjustment.
I once used a speedo to lead a group through and off exercise, funny how in the last 10 laps or so everyone stated getting all serious, getting even lower on the drops and chewing the stem as it 'got faster'. Except it didn't get faster at all, I kept it at the same pace throughout, but because the riders were getting tired they naturally thought it was getting faster.