Saeviomagy
Adventurer
Ridley's Cohort said:
If cannot trust your fellow player to play the cohort such that this NPC carries his weight, I do not see why you trust him with his own PC in the first place.
Which is entirely an individual case. Suppose we're talking about an evil campaign, or indeed any campaign other than the "Hey guy I've never met before, lets go camp out and risk our lives!"
Simply put, the party is paying XP in order to recieve a benefit which they may never benefit from. And they have absolutely no choice about whether they contribute.