If you look at top playoff teams, most of their scoring tends to come from the top 2 lines, once in a while a bottom six player will have a "lucky" stretch (see Smith-Pelly and his 7 goals in 26 games v 15 in two regular seasons), but basically you want the bottom two lines to check and play defense and create a few goals.
This is no shock, what makes a top team is usually 3 or 4 elite talents on the first two lines.
The Flyer advantage is their depth of offensive talent on defense, if the bottom two lines forecheck well, they create opportunities for defensemen to score.