Good question - I have bought the battery. In off-tarmac events you can indeed be on and off the bike frequently, intentionally or otherwise
But it would be nice to have a back-up, because the main reason I've bought the vest is to use it in MCC trials, the next one being the Exeter on 7th/8th January when I expect to be in the saddle for about 15 hours. It won't be warm, so I'll be running the vest at a higher setting. Will the battery last?
One answer is to carry a second battery but it's another thing to buy and carry.
Cheers,
Ian.