Long story short; Boss Soul consumption needs to be stronger. All it grants is some rather...miserable amount of souls in comparison to what the boss provides. For this reason it is best for players to hold on to them and so that alternative option is never really there..."You shall obtain the Boss Soul and the power that is beyond human imagination"
So I think that Boss Soul consumption should do 2 things:
1) Fully heal the player
2) Provide a super buff
Now, what I mean by super buff is something that is really deadly. Big buffs that justify that 1 time usage and give players to reason to consume boss souls for that edge. For example:
Smough's Soul would provide a regeneration of 50HP per second for 1 minute.
Queelag's Souls would increase pyromancy damage by 50%.
Sanctuary Guardian's Soul would provide 80% resistance to lightning for 30 seconds.
...and etc, etc.
What do you think? Would it break PvE if everyone just stacked these and then just let loose all that power on the final boss? Or would this be a great new "tool" for the player?