Theory popularized by: Most teams outside of Philadelphia.
The numbers (examining the 52 Super Bowl teams since 1978)
Only 11 teams have won a conference title game on the road over a team with better record ('79 Rams over Tampa Bay, '85 Patriots over Miami, '86 Broncos over Cleveland, '88 49ers over Chicago, '90 Giants over San Francisco, '92 Cowboys over San Francisco, '94 Chargers over Pittsburgh, '98 Falcons over Minnesota, '99 Titans over Jacksonville, '01 Patriots over Pittsburgh, '03 Panthers over Philadelphia).
Historically, home field has been vital, although six teams have lost a conference title game on their home turf since 1998.
Theory applied to 2004 playoff teams
Seemingly good news for the Steelers and Eagles, except ... Steelers and Eagles fans know all too well about losing at home. Pittsburgh lost AFC title games in '94, '97 and '01 on its home field, while Philadelphia lost the last two NFC title games at home.