From what I understand if you don't hit the eject button and your mech is destroyed, then you die in a horrible fusion-generated fireball. Ejecting also has a delay, so it won't be exactly like every episode of G.I. Joe where guys are ejected before their tank gets hit with a laser.
That being said, if you eject, then you are ejected via booster rockets (I believe) and do not suddenly materialize next to your ride. This means that you will have some time to watch from above, get your bearings, and maneuver on your way down, hopefully into some cover rather than into a carefully timed SRM barrage.