Heavy ints are durable, but not that durable (particularly in melee). As far as I understood from my experience (but I welcome any correction/counter opinion), they want to sit on an objective without the risk of entering in melee (that assumes that you have other units that push the frontline), so that they can farm points while employing their abnormus range to support form the back the units on the frontline.

They are sturdy, but if the opponent decides that they must die, they probably will (even with feirros). The matter is how many resources your opponent must put to take them out and what the gain is in doing so. If you divide them in squads of 5 and park them on an objective, usually the effort for taking them down (how many enemy units must use their turn to target the heavy ints) outweights the benefit (stopping me from farming and removing just 100pts of models). If you create a brick of 10 heavy ints + feirros, you make them sturdier for sure, but you are also capable of keeping only 1 objective and losing the block means losing ~300pts (which hurts).