How on earth can anyone be against more player choice? As has already been stated, those who want to play the PvP game would be able to activate their transponders and let everyone know that they are up for PvP. Those who don't, won't. Everyone is happy... except of course those who just want to... but nah, surely that couldn't be the reason... could it?
But really, it isn't so much about this, or groups or solo. It's about creating that extra edge, that REALISTIC edge of not quite knowing if that guy over there is a Players... sorry 'Pilots' Federation member or not. So yes, the out and out PvPers can storm away at each other with their transponders on. Those with their transponders off are playing a sneakier game, an edgier game... feel those hairs on the back of your neck rise.

So no, let's not limit player choice, please.
Edited to add: Oh and have you thought of this? The ability to have these transponders might just entice more players back to open. Something to think about.