If your company can deal with an employee getting hit by a bus / winning the lottery and quitting, then it can deal with an employee taking maternity/paternity leave.

What about the case where the guy leaves because is cool in software engineering to change jobs very often? or because now language X is cool and the guy wants to add that on the CV and not your old boring language Y.

In any case your employee can leave so make sure you are not dead because most of the value is in this person head. Though you might as always make a counter offer and pay him much more if he is so valuable.