I sent Delta an FYI about this (I hesitate to call it a complaint) last summer. I moved up into EC on an FCA-MSP flight only to find that the GAs had used most of the rest of EC as a free block of seats to relocate three families with small children and two unaccompanied minors.

I wasn't really upset as I had good headphones and wasn't planning to be productive anyhow, but I was sharing my row with a woman who had actually bought up to EC from coach and I think she had every right to be upset (she wasn't, but not everyone would be a good sport about it). After all, the section they had advertised as buy-up seating where you could rest easy turned into a very loud area where it would've been difficult to do just that.