Well i can only speak for myself but i don't go around starting up random conversations in games. Its usually when i'm seeking information, help, or trying to coordinate something. Obviously these interactions can turn into making new friends down the track, but i'd hazard to guess most interactions between players begin with them helping each other in some form for example bumping into one another trying to complete the same mission. I can't think of many situations off the top of my head in which proxi voice couldn't be replaced with a simple invite to voice lobby system.
I feel proximity voice chat is primarily useful in situations when you need to co-ordinate with a random nearby player(s) where the level of coordination is too complex for typing or your in a situation where you cant just stop and type stuff out. If there was an easy 1 or 2 button press way of inviting a player in your proximity (or a group of players) to voice chat that would cover most situations like this, while avoiding the headache that proximity voice could possibly introduce.
If you needed to talk about something general or open ended, for example trading or trying to buy something where you would not know the specific person you need to talk to. I feel like that would be better left to the in-game text chat box anyway.