This idea has been raised numerous times, sometimes by people who have put lots of thought into it, and sometimes by people who have not. Anyway, it deserves an answer.
I've been working on Bivouac full time since 1998, and over that time, have tried all sorts of different advertiser models. The problem is that it takes too long to explain to advertisers why they should pay. I have spent hundreds of hours talking to advertisers, and its just too expensive. The time is better spent improving the website and talking to paid members.
Backcountry exploration and mountaineering is too much of a niche market to get to the size of great websites like wikipedia. There are numerous free alternatives to bivouac, run by intelligent people, but they have been unable to get the quality of Bivouac. They get tons of input, and are easier to post to than Bivouac, but the problem is that in the end somebody has to sort out and coordinate the thing, and you can't do that for free.
You might note that in the past year, many of the major newspapers have realized that they must go to a paid subscription model. Eg: New York times and Globe and Mail are 19/month which is over 200/year. They have finally found they just can't do it for free. Now I think they should lower their subscription rate to something like $40.00/year. But maybe I'm wrong. Someone will find out.
I've been doing this full time since year 2000. There is a huge amount of info, put in by thousands of volunteer hours, and I spend at least half my time helping members, having less members means I can really concentrate on the ones who are serious about supporting the Bivouac encyclopedia.
I'm getting new input as fast as I can properly format it and index it.