Can you prove God doesn't exist? Can you prove that pink bananas don't grown naturally? Negatives can't be proven.
However, here are some links of anecdotal evidence, and articles.
http://www.pistonheads.com/gassing/...eeded+following+accident+with+a+cyclist&mid=0
Admittedly it is rare, but it can and does happen, that a cyclist is at fault in an incident.
Rare enough not to be compulsory, as I said I do NOT AGREE with compulsory insurance. But if the cyclist does NOT have the ASSETS or CASH to cover a claim, it may be wise to get it voluntarily.
I have insurance, not for the primary benefit of other road users. But, to protect ME from any litigation, as I do not have several thousand lay around to cover a possible claim should I make a mistake.
I honestly don't understand why you think it is so terrible that a cyclist who cannot afford to pay for claims themselves, should consider insuring themself against it? Please explain your viewpoint that no cyclist should ever get insurance? As that is the viewpoint you seem to be taking.