I actually wish you could whisper to party members mid-instance as you'd be able to give advice/suggestions in private, and I feel like that would typically be better received than just broadcasting it in the party. On the flip side of course, you could get the prats who then take to /whisper insults, but I would hope that would be the minority of people.

If you have a specific friend, or a small group you want to be able to chat with always, you could make a linkshell or crossworld-linkshell just for that small group.