It may have been lost a little in recent years, but history is vital to the authenticity and legitimacy of F1. It's just more 200mph adverts if you don't have a link to what happened in the past, and nothing has been around for as long as Ferrari. Even Bernie Ecclestone recognises that fact - he would never have done the $80m finance deal with Ferrari if it wasn't the case. F1 would be just another racing series without the history, and with no Ferrari AND no Honda, that history only goes back to 1966, not 1950...and even then McLaren have changed so much in the last 43 years as to be unrecognisable as, and utterly unrelated to, their original incarnation (unlike Ferrari, Honda, Williams & Renault, the four other "old" teams).
Just to be clear, I find supporting a team a bit odd. I never have done, anyway.
EDIT
Someone else agrees too: http://uk.eurosport.yahoo.com/22012009/23/ecclestone-only-team-miss-ferrari.html
EDIT 2
@ragnar "On the comment that McLaren have changed so much they're unrecognisable; the whole sport has changed that much its unrecognisable."
The comparison was with the Ferrari and Williams teams - which have changed remarkably little thanks to a) Enzo Ferrari handling the FIAT buy-in very carefully and b) Frank Williams still being in charge of the team he created; and Honda and Renault - which may have changed the level of professionalism but not their company identity or corporate structure and are still recognisable as the teams which joined the sport. The comparison was not with the sport in general.
McLaren have changed drastically since 1995 (Mercedes original buy-in), let alone their other mutations in 1988 (Gordon Murray revolution), 1983 (TAG buy-in), 1980 (Project Four doing their reverse buy-out of the nearly bankrupt F1 team).