I have some negative and positive thoughts on the idea... it should definately be left as an option for map makers, but I'm not sure out of the box it should be an option.
The fun of tribes is not about guessing whether the person a mile infront of you is a friend or a foe. Tribes is already an insanely complicated game, especially when compared to the likes of UT, Q3, or CS. Adding the player indicators helps to simplify an overly complicated game so at least even in the most hopeless situations, you'll be able to quickly identify friend from foe. By clearly labeling enemies, fighting becomes much more common. How many times have you played a match where you were skiing to the enemy base, and you pass an enemy skiing over to your base? With the red indicator present, a fight is much more likely to occur, and well... fights are fun.
No matter how much IG, you, or even Dynamix want(ed) Tribes to be an insanely team based teamwork game, forcing such teamwork elements like keeping a sensor up is just plain not fun, unless you are a super fan of defense. We already have inventories, turrets, and generators that we should be paranoid about protecting when rapes occur. This just adds one more elements to that list. Without invetories or turrets or generators, you can still fight and see the enemy and have a ball. But without sensors, you even lose the motivation of fighting the enemy. You know as well as I do that if you see two people, one without an indicator and one with, you'll go for the one with first. They are more threatening, hence more motivation to fight.
This kind of leads to my other point of TKs... I'm sure you've thought about it, but with indicators, being able to tell who is a TK and who isn't is quite easy. You know that the person who is tking can clearly tell the other person has a green thing over their head hence they are purposely doing it. Without indicators, I see TKs happening more often accidentaly (which could cause false accusations and kicks) and the people that really are TKing could get let off the hook more often ("there wasn't an indicator! I couldnt' tell..."). I dunno I just see this as presenting a problem.
Other than that, maybe this whole sensor idea should be gametype based. In a CTF map I don't think I'd particularly enjoy it in the least bit. There are already too many things to focus on. But on other gametypes it could be fun... I couldn't really say cause I havn't played a lot of the new ones though.
T:V is a team game with jetpacks and guns, but that doesn't mean teamwork should be forced on a player preventing the cowboy from killing his fair share of enemies. If you guys want a larger audience, focusing the teamplay more tightly won't help. Bf1942, UT2k4, Americas Army, CS all have teamplay elements, but you have to admit that the majority of all those players only pub and are to a large extent cowboys. They wont' want to pick up T:V if it hinders their ability to cowboy. Tribes1 and 2 were filled and still are filled with cowboys that work for themselves in order to help the team as a whole, and I just see this sensor thing as taking away from the whole "random duel in the middle of the game" occurence. For the teamplay lovers, this idea is a godsend, but the pure teamplay finatics are few compared to the majority, though they are quite the vocal ones. Adding this sensor setup will either increase teamplay or make the cowboys mad. I forsee the latter.
I'm big on teamplay but when I'm on a pub, I like to cowboy. I think this is an extremely intruiging idea and honestly if it is implemented fully, I won't be too upset and it won't keep me from playing the game. I just think it could hurt the game in other ways and bug a lot of other people, and even bug me at certain points during the game.
This might be one of the things we should wait till Beta to see whether it's good or not.
EDIT: I realize green IFFs will still be in place even if the sensors are gone. I just have a hard time trusting that when I shoot someone without an IFF, they won't be friendly. I need identification! haha...