I always used a piece of eggcrate in my cichlid tanks to protect the glass bottom from falling rocks. Is there any reason to NOT use eggcrate in a planted tank?
That said, anytime I'm using extra large rocks in a tank, planted or not, I'll put down the eggcrate. Usually, I use a fairly thick substrate bed so that the roots have to go deep to really get intertwined down there. Most of the time, even if you rip them out, you'll still be left with enough roots to replant.
Not one bit in my experience. If you make the meeting at my house next weekend, you can check out my 75G for yourself which has eggcrate in it. Growth is fantastic.