you calculate a single engine net take off path (or more likely a bit of software does it for you) then fly an all engine operating take off and you don't do it the way the manufacturer intended. You hold a take off flap setting, climb against drag, make more noise than necessary, end up in an unknown and unprovable vertical situation with regard to SID gradient, and you use more fuel. And there's no reason for it.
I'll put 100 quid on it. Does that help?