A cycling friend of mine, who is well versed in financial things, only insures what the law requires him to. Over a period of time he reckons he saves oodles. He convinced me.
I don't insure my bikes. I gave up doing so when the mustelid tendencies of the insurance industry persuaded me that the chances of being properly compensated for any losses would, in reality, be minimal. To do so in a way that they were properly covered would cost a new bike's worth of money every year. I keep them well secured, and in several locations, when not being ridden. When out, I strangely only usually ever have one with me.
I never buy "a bike" - it's all e-bay and other purchases, so I'd have a job "proving" what the value of anything lost was. A "new" bike like one of my race bikes would be a lot of £000's. However, if one was nicked I could replace for a maximum of say £2,500. "Good" insurance for my bikes might cost £1500/annum (yes, I am serious, that's only 15 bikes at £100/bike - BC insurance, which is about the only one that offers realistic cover, is at least that). In 2 years I am in "profit".