I try to look at it from an ROI point of view...

Can you get the weaker members up to speed and therefore raising their productivity before whatever product/goal (or any future similar goal) is reached for it not to be of benefit? Basically, if the weaker component of the team still won't be able to pull their weight and produce like the rest then surely it'd be a good business decision to get someone better - as that person (or those persons) have been hired to do a job that is not being completed and therefore aren't adhering to their side of the employment contract?!?

Sadly, being a human being means that decisions are often made with some (if not a lot of) emotional influence and that's when decisions can become 'irrational'. It's at this point where personal preferences and conflicts can cloud the right business decision.