I'm trying to assess how important side visibility is at night on a bike.
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Summary: very few. Somewhere between 20% and 90% of crashes involve a motor vehicle approaching a cyclist side-on (guess ~50%), and somewhere between 30% and 90% occur during darkness (guess ~50%). Unfortunately 40% or more are the result of drug-impaired driving or riding. So perhaps 60% of that 50% of 50% = 15% could be avoided if the cyclist had bright side lighting.
The wide ranges are a result of small numbers and inconsistent statistics collection. As with anything that is quite variable and happens infrequently, random variation makes it difficult to see trends.
First, modern usage is to talk about "crashes" or "incidents", partly because that allows us to talk about who was at fault. 99% of crashes are caused by people who intended to do what they did but failed to do so safely.
New Zealand publishes something close to what you seem to be after: Cyclist-Crash-facts-2012.1.pdf (pdf link) that includes this summary:
BicycleUniverse has a useful summary with references:
Although they list a number of conflicting statistics on intersection fatalities (also 89% and 33%).
It's rare to find that type of incident breakdown in published statistics. This page from the USA, for example, doesn't even separate fatal from non-fatal accidents when attributing fault. And they talk about cyclists breaking the law but not motorists, so (for example) if a cyclist did not have a rear reflector on their bike when a motorist T-boned them the statistics would say "cyclist breaking the law, hit by car". This is regrettably common across all analyses.
The key points there are:
The first point is near-meaningless (75% of cyclists were killed during 66% of the day). But 64% not at intersections suggests that side visibility is not likely to be a major issue (at most 36% of fatalities).
There's a big collection of links to papers at the Cycling Resource Centre (Australian site, worldwide links).
There is an excellent, although complex, chart on page 27 of "Road Traffic Crashes in NSW 2011 (Australia)" (pdf link) but it covers all road users and does not break out cyclsis. I suspect what you're looking for is something like this that only covers cyclist incidents.
For non-fatal accidents, BicyclingLife compares three cities (with references) and concludes that motorists failing to give way at stop signs in the most common cause of crashes.
You have an excellent answer to the question as asked already, but even if the answer is "very few" there are some things you can do to help yourself at such little effort that they're still worth doing (IMO of course). They may also help with scary near misses which of course aren't logged, and it's also courteous to be clearly visible so long as you don't dazzle anyone.
I'm impressed by the reflective sidewall stripes on tyres like Schwalbe Road Cruiser and Continental Comfort Contact even when quite muddy - try walking towards a bike with them holding a torch. And they're good commuting tyres anyway.
I've also got light up valve caps and use white/translucent little silicone LEDs on the offside fork and chainstay or rack. The former might be just a gimmick - but they show up a long way off, the latter show red at the back and white at the front anyway,* with the diffuse light at the side an added bonus
My bike also has retro-reflective paint on parts of an otherwise black frame - again, even dirt-spattered it's pretty obvious when a light shines on it - I don't know if there's anything easy to retrofit.
*I advocate multiple steady lights front and especially rear - I used to commute by car on country lanes in the dark, shared with bikes, and 2 steady rear lights meant I could instantly tell how fast I was gaining on a bike, and get a good idea of how far away it was.