Do I ride with an SRM, Ergomo or Hub now... Nope. Not even a computer.
At one point in my riding, I used both SRM and Tap, as well as a computrainer and they are absolutely a tool that gives results as part of a training program... So is the Ergomo.
That power measurement devices and even basic heart monitors are valuable tools when used effectively is not debateable. They quantify and record performance and allow for accurate, repeatable measure levels that are the basis for performance growth.
Wether or not they are a waste is completely dependant on rider training. For most of us here, every electronic device we strap to our bikes is simply an accesory and power meters are just the next level of onboard entertainment / vanity.
This leads to one of my points in sort of a round about way. I think it is good that everyone get out of cycling what they wish & that they enjoy it. If motivation is derived from a power meter or a computer I think that is great! I think everyone has a method of becoming a better rider that works for them.