I guess I'm a low value customer as well-- Diamond since the beginning, just a blue DL Amex card that I don't use because I get better value from other cards. I get to the DM MQD with at least 1,sometimes 2 AF/KL/MU etc trips to top out my spend.
But I feel I am well-treated by DL -- when things go wrong I get what I think is relatively generous compensation -- vouchers and/or miles. Been allowed to change a few trips without penalty when there is a good reason.
The only thing I think I've missed out on are Porsche rides -- I've only had 1 although it was pretty neat to drive around JFK (T2 RJ maze to T4).