To promote teamwork, Infinity Ward decided to disable party chat in certain game types, I guess hoping that people would say 'Ah well, might as well talk to everybody, I guess.'
Personally, it annoys me to no end. I hate listening to 10 years kids ramble on calling each other names that I can't say on Yahoo Answers.
Basically I do the exact same thing as you, mute everybody on my team and just 'pretend' I'm still in party chat. Although, if you're only playing with one other person, Private Chat is never disabled, so you'll be able to jump in there.