Nick, I think your view of BMC membership is a bit awry. This from the 2010 Annual Report:
"There was good growth again this year in individual membership which meant we
finished the year with a combined total of 73,956 members, an increase of 2,844. The breakdown of this is 49,456 individual members (an increase of 2,604) and 24,500 club members (an increase of 240). The number of affiliated clubs dropped by 9 to 288, of which 48 were student clubs. The number of club upgrade members (club members paying extra for individual member benefits) was down slightly to 1,578."
As to what BMC members do, here's an extract from the Member's Survey carried out in mid 2010:
"2.12. Members indicated their three primary mountain-related activities. The most popular activities were hill/mountain walking, rock climbing and indoor climbing. In total 86.6% of members indicated hill/mountain walking as one of their primary mountain-related activities. 61.7% had hill/mountain walking as their primary activity. Several members (12.5%) only selected hill/mountain walking as their primary mountain-related activity and did not select a second or third activity."
As to why people join, I don't think finding people to go walking/climbing with is high on the list (as you say, you'd go to a local club). Again from the survey:
"Individuals tend to join the BMC to access travel insurance, register for mountaineering awards and to support the work of the BMC. This is somewhat different to the reasons for membership renewal as existing members mainly rejoin to support the work of the BMC and to benefit from the travel insurance."
So a lot of it is about supporting a representative and campaigning organisation. The BMC always include "climbers, hill-walkers and mountaineers" in any statement about what they do and who they represent (astutely you might think, because hill-walking is where the volume that a campaigning group needs is to be found), but then the bulk of their efforts seem to be directed at the climbing and mountaineering elements.
I guess any efforts they make to change this balance are to be welcomed (imo), but it does sometimes seem like pretty slow progress.