I agree with you in principle, and have also found the use of party chat annoying. However, I totally see the utility of it on the DM side as I think you do as well, especially when there is just one DM on who needs to be multiple places as once. To deal with the "spying" issue (which as you know from past servers I am well aquainted with
) Just do what Mikayla suggested. Send the DM a tell "I'm spying can you please ask them all to drop party chat (without telling them I am here) ?" I am guessing the DM may ask you why you are spying which will then hook your PC into the plot which will then make it more interesting for everyone involved. 
While we are on the subject of party chat, there is only one other time I use it as a player tool. Playing a ranger I often scout ahead of a party. Once you are out of the normal chat radius your party can't see your emotes in the chat dialogue box any longer. So I will switch to party chat if I want to emote *Waves them over to his position* or *Holds up a hand signaling them to stay put* . I have also used party chat when scouting through a transition such as a cave mouth. Something like *calls back through the cave entrance that all is clear* As long as you are within line of sight or hearing I think this is a totally appropriate use of party chat to make up for limitations in the game mechanics.
