I couldn't agree more. Parties should only be matched with other parties. If its 4v4, great. 4v(2&2), that's fine too, though I'd say look for another party of 4 before getting 2 groups of 2.
To add to the problem, you can't even see that they are in a party until after the game. In Reach, you could pick out who was in what party (there was always a connected line next to their names). It was subtle, but it was there.