A speedometer is fun to look at on downhills, but has no real use otherwise.
When training for racing, I would use a power meter for specific workouts, otherwise I found no need for speed or distance information.
Effort & time are important.
If you're not "training", then powermeter and everything else are just blinking numbers.
At least with a power meter you can see numbers directly related to you, not influenced by weather, tire pressure, terrain, bike setup etc...
If you want, you can push yourself a bit in a more meaningful way, such as "Maybe I can finally hit 800 watts on this climb ..."
Last edited by Homebrew01; 05-10-15 at 07:31 AM.