Use rechargeables. Saves a fortune and they last longer in the lights 'cos they're higher capacity.
I agree with you. Batteries should say what capacity they typically have, in the form - eg: 900 mAH at 250mA discharge to 1.2V when new.
OK it would be a bit less with time, and at a typical disharge rather than for a particular bike light, but if all the manufacturers had to quote to the same discharge for that size of battery at least consumers could see which one would keep their little pink rabbit moving the longest.