the point of a planetary bombardment for most wouldn't be the extermination of all life on it..it could be but that would probably be rare.its a gross and careless expenditure on munitions. the point would be to destroy as much infrastructure as possible. to potentially bomb that back into the stone age. life would still be there. it would just be very difficult and radically changed. ur not going to waste munitions on habs and the sort...u lay waste manufactories that produce essential products, power generators, processing facilities, communication hubs
and everything that i have encountered in WH40K states that while Exterminatus is a viable option it is NOT common nor commonly known. it is a form of ultimate sanction when all else fails. the order can only be give by a Warmaster or an inquisitor. and even then thats dicey,,Inquisitor Kryptmann was declared Traitoris Extremis for his destruction of a number of worlds to stem the Tyranid advance.
to my knowledge there are 3 methods of performing Exterminatus 1.a viral barrage AKA the 'life-eater' in the HH novels,2.Cyclonic Barrage and 3. the atmospheric incinerator barrage
