The system you ask for would be very good, but it would indeed render C3 rather useless.
You think so?
So why doesn't this?
There is also Teamspeak.
Or this?
Just type "ctc C4" where C4 - any sector number.
Lots of obvious reasons will not render C3 obsolete (distance, exact location, movement of the target, damage information, friendly Longtoms, ...).
You can also use something like "ctc C4.5" where ".5" is "keypad 5" or a subsector in C4, divided as the keypad goes. But such precision is rarely needed.
A single sector can be pretty big, up to one kilometer should be possible. Dividing it into smaller parts when relaying information should be necessary in nearly every case - after all, 'Mechs are very small entities in comparison.
@ Poldi: I think somebody may have brought this idea up before. It may have been a little more complicated (a tactical map for the leader of a lance maybe), but I am sure I have read this before. And I suspect we won't see it before 1.1.
PS:
The marker should slowly fade or change colour, so an experienced player will be able to see how recently an enemy was spotted and how useful the information still is.
PPS:
Beginners will spam this feature, willingly or not. It is actually quite similar to TAG, isn't it?