Its obviously not just because of grit. These are energy lines that can give some goal support, on good teams anyway. The teams that you mentioned are not good examples for your argument. The lone-non gritty player on those 4th lines is Chris Versteeg, and he found himself scratched a nice chunk of time in the playoffs. None of those teams you mentioned have young talent on their 4th line. 3rd line, yes, but definitely not the 4th line.
Rangers:
Boyle, Moore, Dorsett
Ducks:
Palmieri, Bonino, Winnik
Kings have two lines in the Mold of a good 4th line:
King, Stoll, Williams
Clifford, Richards, Lewis
Blackhawks:
Bollig, Handzus, Versteeg
Bruins:
Paille, Cambell, Thorton
Maybe in theory it makes sense to have a young skilled guy matched up against the lesser skilled gritty lines, but in practice, obliviously not so much. The match-up just doesn't work that way.
The reason the Devils have missed the playoffs is a lack of scoring depth in the top 3 lines. Why a 4th line is being blamed for a lack of scoring, when a 42 year old Jagr is your best scorer, is beyond me.
The AHL arguement for Gionta is also ridiculous. I guess anyone who plays there into their mid 20's is useless.