Access to BHX
I agree pwalhx, access works both ways, being easy to get to means it is easy to get from. Although BHX is my closest and preferred airport (and is just 25 mins away by direct train from Leamington where I live) as a solo traveller with no kids in tow, I can drive to Luton in just over an hour (on a good day mind!), take a direct train to Manchester, with a simple change to get from Piccadilly to the Airport and take a direct train to Reading and get a coach to LHR. Although all take far longer than 25 mins they are very easy journeys and I have done all of these in the last 12 months to catch flights from these airports. With kids and lots of baggage however I would agree that these are not attractive options.
BHX suffers from the fact that people living on the 'edges' of its 'catchment' area are as likely to use EMA, MAN, LTN as BHX. Despite the fact there is a direct flight BHX-TXL I have a friend who lives in Rugby and is flying Easyjet to Berlin this weekend as LTN is just as easy to get to from there (Easy were much cheaper too which swung it). I'm sure other airports must also suffer like BHX losing pax to other airports nearby but perhaps not as badly?