Sure, airlines have good reasons to want to fly into Heathrow, most certainly British Airways and the rest of Oneworld airlines, as it is an important hub for them (in 2007 47% of BA's traffic at London Heathrow was connecting).
Connecting traffic at Heathrow is also important for non-Oneworld airlines. CAA data for 2007 shows how of all airlines carrying over half a million passengers at Heathrow, connecting traffic was over 15% of their passage in all cases except in three (Swiss, Air France, and KLM, with Alitalia just above this mark). As expected, since they did not have a local partner, connecting traffic at Heathrow was relatively less important for Skyteam carriers).
I am not sure these numbers would still be valid for Star Alliance's carriers though, I guess that after BMI's demise and absorption by British Airways its share of connecting passengers will fall to Skyteam's levels.
Connecting traffic involving non-aligned carriers was 3.5M, however, 1.4M involved a Oneworld transfer, another million involved Star (most likely BMI, that is now gone) and transfer traffic between non-aligned carriers at LHR was less than one million passengers in total. These might still sound like big figures, but they are the aggregate of several tens of non-aligned airlines flying to LHR on that year and I suspect the Gulf carriers Emirates, Etihad, Gulf Air and Qatar Airways are responsible for a significant part of this traffic.
So, even if you are not in Oneworld, you can still benefit quite a lot from flying to an airport with lots of connections (I must say the connecting traffic figures for non-Oneworld carriers are higher than I was expecting...)
But is this extra traffic enough to justify the London Heathrow premium? I don't have enough information to be able to answer this question...but if you rely mainly in origin and destination traffic, and that means basically, people that start their journey in central London, I wonder why London Gatwick does not emerge as a more serious competitor to London Heathrow.
Will Gatwick airport finally get the credit its deserves?
I have been using London Gatwick a lot this year, traveling in and out of several locations in central London and I struggle to find the reasons why LHR would be a better option for a passenger that has London as the start or end point of his journey and that is not flying with a Oneworld carrier.
LGW might be slightly further away from London than LHR, but the difference is not huge (although maybe just far enough to discourage those that get to the airport by taxi?). LHR has the tube and the (in my opinion) overpriced Heathrow Express, but LGW has several fast mainline train options, with frequencies of no more than 15 minutes, that can take you to several points in central London (Victoria, Charing Cross, St.Pancras International, London Bridge) in about half an hour. It might be slightly more expensive than the tube and 10 minutes longer than the Heathrow Express but I think it beats both hands down in terms of value.
My guess is that more and more people will warm up to the charms of Gatwick airport as pricing becomes more of a tool for rationing airport capacity in the South-East of England, at least until it solves its chronic shortage of airport capacity and a final decision is taken about the future of Heathrow (to check the different options, check this piece that was recently published on Flightglobal).