Highly situation dependant. Defile deals double dmg if it hits 2 targets, so just for that situation it would probably be worth it. Delaying it 15 sec every time by 15 sec for an add spawn? Again depends on the dmg gained vs dmg lost by defile uses. Also need to keep in mind that defile is still a trade and not free, so if you lose on a defile you lose defile dmg - SS dmg and not the whole defile.
Additionally if you use defile on Multi target you increase the percentual gain over using SS for it, as SS dmg does not scale with targets.
Shouldn't be too hard to calculate, as you simply need fight length, add spawn rate and the time the add lives per spawn. You look at synchronisation of defile and add spawn over the whole fight and the dmg it does and look at the scenario where you delay defile for every add spawn, if possible and add that dmg up.
Aside from scabbard we also have barely any cd which influence defile dmg drastically as unholy and as scabbard lasts longer than defile and is a flat gain this is also not too hard to add up and might not even matter much on average, since its natural line up would be every 3 defiles and allows some delays due to its duration.
Breath is a similar story, although the gain from another target is smaller and gets even smaller for every other additonal target so that in the end correct line up with CDs over the whole fight is likely to be more important due to its CD being much higher than defile.