That's where you could be wrong - an annual policy against car hire excess is £49, so it's not that expensive at all, covering every driver named on the rental agreement, and including theft, fire, and all damage, including undercarriage, windows and tyres. Many of the 'top-up' insurance policies you buy at the desk don't actually cover you for all that - they often just reduce your excess.
If you hire a car more than once a year, I think that it's worth it to take out the extra policy. Do be aware, though, that most of these policies won't insure you if you rent within 150km of your normal place of residence, and some (eg Worldwide) don't cover you for hire in your entire country of residence.