This is more about what we see as the future of cycling. I don't want everyone to be a 'really keen' cyclist. I want people to see bicycles as normal every day tools. Putting a piece of polystyrene on your head gives the impression of a dangerous unusual activity and less of a normal everyday item.
Cycling is a relatively safe activity. You are more likely to injure yourself per hour gardening or per mile hiking (source CTC 2012, NIH 2006). Most head injuries on the roads are actually suffered by motor vehicle occupants (why not wear helmets in these).
In studies on countries where compulsory helmet laws have been enforced, cycling rates have gone down massively, yet the risk to cyclists per mile has actually increased. I've just spent a lot of time on a stall promoting cycling at work where we have a mandatory helmet rule. We were asking people why they didn't commute by bike. The helmets are number two on the list of reasons why people don't commute behind thinking it would be too dangerous and ahead of a lack of shower facilities. Instead people sit in cars in a massive traffic jam half-way across West Cumbria twice a day.
Promote helmet wearing however much you like, I'm happy for people to wear them if they want and will try and help people pick sensible helmets for themselves if asked and help with fitting of helmets. Currently the law allows you to wear one if you want and not if you don't want. I fail to see why the pro-helmet advocates are insistent on enforcing their world-view on everyone else?