c'mon people...how many mega bosses are there that spawn alongside goblins ? And most of the times you use a heavy , you will have some dps to clear the enemies.....and goblins are really easy to kill. Hell , even assuming no other dps is placed , the heavy can kill the goblins pretty fast because they are weak.
The only situation where goblins would be alive for a longer period of time , it would be if the first blocked unit would be a boss/megaboss....and honestly those don't tend to spwn before goblin spawn.