WHBM - Yes I have had similar conversations with people travelling on the LHR-MAN service which I have used regularly for several years. Many of them living in Bucks (like myself),Berkshire & Surrey do not want the hassle of trying to get into either Euston (really only accesible by public transport / no parking) or across to Watford Junction (with its lack of car parking spaces - even early in the morning!), compared to the numerous choices of car parking alternatives at LHR.
It may not be very green but it's certainly more convenient!