back ups would've great, but the space on my handlebars is stupidly limited - it's a cyclocross with extra brakes on the bars, so it took me ages to find any light that would fit at all!
However, you speak the truth, and I'm going to have to buy another small fitting light (probably another flea tbh, unless anyone else knows of a light with a narrow fitting, where the bulky screw part of the fitting is at the rear of the bar) to leave on as a spare.