I pointed out that there is no need for a seeming disadvantage to be offset by some redeeming advantage.

You could come up with any number of possible reasons if you were looking for a "reason," although it could simply be a side-effect of some other mutation (such as the layout of the pelvis, intestines and abdominal muscles being more efficient for hunting/etc. without the "extra" parts.)

Here is another important inversion that you seem not to have considered, though: do you think that when externalisation first happened, testicles were as sensitive to pain as they are now?