A Question About Marginal Hands

[PokerProdigy]

I have heard and read many times that most of your profit in limit holdem comes from marginal hands. Intuitively this makes sense because the good and bad players wouldn't (usually) win or lose any more or less with the big hands like AA, KK, QQ, JJ, AK, etc... I used to always try to play a very tight preflop game in limit holdem (like the one in GSIH) and try to stay away from marginal hands. I am wondering if this is a bad idea, especially since I have read SSHE a few times along with many other good poker books, so I feel like I have an ok grip on the game and how to play marginal hands.

So my questions are:
1) Do you agree that most of the profit in limit holdem comes from marginal hands?
2) Do you agree that it is actually a bad preflop strategy in limit holdem to play too tightly preflop, because if you don't play enough marginal hands it will be difficult to make a profit?