The MH370 777 was fitted with ADS-B, which is how the various internet based tracking services managed to track it. Unfortunately, it was so far away from the nearest receiver site that the line-of-sight needed for microwave transmissions wasn't possible below 30,000 feet. If the plane made an emergency descent, it would've vanished from tracking very quickly though perhaps not as quickly as it did vanish. AF477 sent ACARS messages over its satellite communication link, as I recall, these are not sent as frequently as ADS-B messages but would be enough to locate the aircraft very approximately. The problem with any technical solution is it may not continue to function for the whole flight, which is why ELTs exist, but they're not indestructible or guaranteed to be in a position to transmit after a crash. There is only so much you can do with electrons, when you're facing the prospect of a large metal object falling 35,000 feet into salt water.

ADSB is a whole new can of worms [0] just waiting for people to exploit…

[0]: http://dangerousprototypes.com/2013/02/28/defcon-20-renderma...