I think personally it's always worth it, unless, as RR999 says, you can have a fair bit of money (I'd not want any less than a grand per bun, ideally more) reserved in case of sudden illness or a reccuring illness. I'd never go without it myself - I pay £7.90 a month for Alfie, that's £94.80 a year which, in the grand scheme of things, is peanuts.
For example - Alfie cost me £700 in one day's worth of tests/treatment. Let's say I'd reserved £1000 for him for the year. If then, two months later, he got stasis and needed hospitalizing and treating, that would be my entire allowance gone. With insurance, I'd have another thousand left. In not quite three years of having Alf, I'd say we've spent between £3000 and £3500 on him, with that cost still rising! Had I not have had insurance, I would have had to put him down as I couldn't have afforded the treatment.
Another of my bunnies cost me £1300+ in six weeks, and even though I had to have her pts, insurance bought me some precious time left with her. To say the year's insurance cost us just £100, it was so well worth it. It's just peace of mind, IMO.
If you're struggling for money, could you change to cheaper insurers?