The reason that you rarely, if ever see a wheelchair user on the London Underground is the very patchy provision of facilities. On one line (can't remember which as I've not got my tube map with me) wheelchair useres can get on at one end and off at the other, not much use unless you just like travelling for the hell of it!
As far as determining the difference between a want and a need, who decides that? Should disabled people only be allowed to travel where there is a pressing need for them to? I have one leg, for walking around an airport it's not normally a problem but standing in queues casues me real problems. From time to time I fly for business. Is this a want, as I chose what I do for a living or is my travel a need as I need to be able to earn a living?
On a more prosaic level, do I really have a 'need' to go shopping? I can shop on line but I prefer to actually choose the veg and meat I buy. Deciding that a certain class of the population has to justify the reasons for them travelling about the country they are citizens of reduces them to second or third class citizens. As a society we may as well go back to the instututions were disabled people were corralled in apalling conditions in the 1920s and 30s, after all, out of sight is out of mind.
With an aging population these access measures will become more and more inportant. It should also be remembered that disablity can strike at almost anytime without warning, all it takes is a virus, a small clot or someone else driving drunk.
As for the carriers' ability to make progress on this, I'm not holding my breath as the airports can't comply (in the UK at least) with the earlier legislation yet.