I offer an "informed decision" answer, partly based on a (slightly skeptical) medical knowledge, and from my experience as a biker. This should NOT be considered some truth emited by a potential authority-figure, neither an advice to act this or that way.
On one hand, there is a very positive association between skin exposure to UV radiation (such as sunlight) and skin cancer. This association is much greater in people who have a very white skin color, such as most european/caucasian ethnic groups. It is believed that this is due to the small capacity of these skin types to produce melanin. Melanin is a dark pigment produced by skin cells to protect the cell nucleus, and its production is somewhat stimulated by sunlight exposure. Please read these very interesting paragraphs from Wikipedia: http://en.wikipedia.org/wiki/Melanin#Human_adaptation
Now, as a biker, my experience is the following:
- There was a time when I didn't ride much often in direct sunlight. I almost always got sunburn when I went to the beach during the summer, unprotected, when it became obvious that sunblock was needed, at least on high-exposure seasons and places. During that time, riding across noon in the summer almost always implied a reddish sunburn on neck, nose, calves and forearms.
- Some time later, I started to use sunblock to ride my bike during the summer, if I knew there would be many hours exposed to the sun. It worked a lot, no sunburns at all, but if at any time I forgot the sunblock, I got sunburned.
- Nowadays, in what my wife (with much reason) considers a very questionable behaviour, I gave up any kind of sun protection to ride the bike, completely. This was due to many "reasons", which I am not ashamed to share:
- I think the skin must be given the opportunity to develop at least a minimum amount of self-defense;
- The anti-cancer pro-sunblock-all-the-time propaganda contains all the tints of a pharmaceutical lobby, at least partially;
- Sheer lazyness, sort of.
The experience I've been having, and it is a totally empirical, baseless, subjective one, is that I very rarely get sunburnt anymore. During the summer, the exposed skin gets very tanned (the classical "cyclist tan") and don't hurt except after very long rides under scorching sun (I tend to use sunblock on these occasions, anyway, but sometimes...).
I think both views (sunblock-always and sunblock-never) are too extreme. That's the reason why a lot of information is needed to choose an "informed decision" strategy, so that one can achieve a reasonable balance between them. The scientifical evidence about cancer vs. sunlight gets to the media very distorted by oversimplification, incompleteness and commercial/professional bias. In the other hand, as already been said, there is an undoubtfully positive evidence of association between UV radiation exposure (as sunlight) and skin cancer incidence on clear-skin populations, which might or might not be overrated.
Hope this helps