Thinking about House Elves

I've been thinking about house elves lately, for some crazy reason. If my memory serves, we've met four of them directly in the Harry Potter novels: Dobby, Winky, Kreacher and Hokey (the House Elf owned by Hepzibah Smith, the woman who introduced Tom Riddle to Hufflepuff's Cup). We met several others in the kitchens of Hogwarts when Harry, Ron and Hermione went down to visit to see Dobby but they play little or no part in the stories.

And we've seen some of their powers:
1. they have magical powers that they can use without a wand;
2. those magical powers include the ability to apparate, even in places where witches and wizards cannot (such as Hogwarts, Voldermort's cave, and Malfoy Manor);
3. when they apparate, they can take a number of other people with them;
4. they are able to disarm witches and wizards at short distances;
5. they can hear their masters calling to them no matter where they are and immediately go to their masters;
6. they appear to be good at tracking people and are remarkably strong for their size.

Although it has never been directly discussed, I believe, however, that the magical powers of the House Elf are limited in many ways. They certainly have never been able to escape the servitude to wand carriers in which they have been placed. Although they seem to be able to combat wand carriers fairly effectively in brief encounters, it does not appear that they could openly revolt against them. They do not carry wands and do not appear to have made any attempt to expand their powers to include wand use or to rival the powers of the wand carriers.

Okay so far, right? So my question is this: if a House Elf's powers are limited but still very useful (note, Harry has Kreacher track down Mundungus, Regulus Black uses Kreacher to help steal the locket from Voldemort's cave, the hero trio certainly do owe their lives to Dobby, who saved them from Malfoy Manor), why does Harry not make more use of Dobby and Kreacher in Book Seven?

I will have to research the question because I think Harry makes some mention of the possibility of using the House Elves somewhere in the middle of The Deathly Hallows but I'm not sure. I would think, however, that they could at least have solved their hunger problems by having Dobby bring them food every day from Hogwarts.

I must be missing something.