This is just my guess:
Everytime your team makes a wish, (for immortality or power) you get one wish point. If your team however, chooses "Friendship", you get an extra wish point added. So your team will get two points from that wish instead of just one (although they get nothing good for the player as an individual). I suppose it's useful if your team is behind in wish points, and needs a boost to get back into the lead. If friendship was not used, your team might have to collect the dragonballs twice, meaning twice the time. If your team is behind and pressed for time, I guess friendship is a good option.
So in recap: When you wish for immortality, one wish point is added, when you wish for power, one wish point is added, but when you wish for friendship, you get two wish points. The match is decided upon wish points, not wishes. You think they'd make a point category that was completely irrelevant to the game? Come on now!