I think you've got it :) (Destiny)

by Ragashingo , Official DBO Cryptarch, Saturday, May 03, 2014, 12:57 (3866 days ago) @ scarab

My speculation is very similar to yours here. The Traveler must be doing SOMETHING that keeps the air clear of enemy ships and incoming weaponry or else someone would have just tugged over an asteroid by now. It's a neat plot point really providing players with a place to always come home to while also giving them a reason to defend the walls and stretch out and strike at The City's enemies. Can't let the Fallen build too big a base in Russia or wherever because while the Forces of The City excel in small raids and Strikes, we probably don't have the out and out firepower to fight back a full scale ground invasion.

It's going to be quite the thing in Destiny 2 when something happens to the Traveler leaving The City vulnerable for the first time in centuries. :)