We're a family of five, so going to Disney is expensive for us.
I don't think there is anything wrong with bringing water or little juices for the kids into the parks, or snacks. I also don't see a problem with people bringing a sandwich or whatever into the park, as long as they don't sit at the tables meant for those dining at the restaurant, without ordering anything.
We do bring snacks and soft drinks to our room for snacking. I don't personally bring sandwich items, grills, hotpots, etc for cooking meals, but only because I'm lazy and I like eating out while I'm on vacation. But if others need to cut costs, I can understand. I do think Disney is aware of it.
But I also don't think that its very nice of Disney to charge the prices that they do for water or some of the foods. For most people, and especially my family, we eat all our meals at Disney (we do occasionally skip breakfast or have a granola bar) - they could give you a little break on WATER!!
But I CHOOSE to go to Disney, and I know upfront what the costs are, so if that's what I'm choosing to do, I pay the cost.