First of all, in terms of raw damage, Mortal Blow is probably the worst 3p source skill in the game. Even if your character is sneaking, it does less than 300% of your base attack damage. The point of it is to be able to kill anyone under 20% vitality, this lets it do a lot more 'damage' against high health enemies. It is not meant as a starting move.
Well, this boss needs to have then more than 5000 HP (on level 15) to "deal" more dmg with the vitality aspect of mortal blow than with the "normal dmg" of the ability. Why? 20% of 5000 Hp is 1000 DMG. This is the amount of dmg you anyway deal with mortal blow at level 15. All bosses I fought so far at level 15 had between 3000 and 5000 HP max. If you then also would aim for the same dmg as the spores then you would need to have a boss with like 7500 HP at level 15 as 20% of 7500 is 1500. If you would aim to make it Source Point equal DMG to the acid spores then the bosses would need double or more HP. Again ignoring the fact that acid spores can hit multiple targets and can be used two times during the uptime of the plant. So the vitality aspect of mortal blow sucks as there might just be only "final bosses" that have lots of HP to make this ability worthwhile in comparison to the plant that at present just uses one SP and deals lots more dmg than that ability and can be used in various ways (even as cannon fodder because of the huge amount of HP).
Edit: And yes, the fact that summoning also buffs non-summoner-summons is broken as the "normal" acid spores of geomancers needs 2 SP and does considerably less dmg. I even would say that skill of the geomancer does the same dmg as mortal blow, so it is not so much about the skill "acid spores" itself but the buff the artillery plant receives from summoning which indrectly buffs the dmg of "acid spores". I am sure no geomancer would love the fact that a summoned creature does with its skill almost double damage than he can do with the same skill whereas the geomancer even needs to spend more SP.
