I have dabbled with leather, with mixed results. On one of my lighter rackets it is the perfect addition on a slightly heavier racket it has a very negative effect.
But I do think its a matter of personal choice. As was the case for me, leather on one of my rackets caused hand and wrist pain so I switched back to stock form synthetic. I suspect the view of the manufacturers is that the majority of recreational players will prefer softer synthetic grips and therefore they make the racket to suit that market. perhaps they feel that anyone adding a leather grip has some idea of why they are doing it and will be prepared for the way it affects the balance and weight, as opposed to putting leather on in stock form, only for the player to get a totally different feeling racket when they switch to synth. Lets not forget that pro's rackets differ vastly to retail versions.
I also suspect that the Federer racket and MacEnroe racket are given leather in stock form as to replicate those used by the signatories. Even though you can't see Federers leather grip, Wilson also market the Wilson leather grip 'as used by Roger Federer', so it would make sense for them to stick it on his retail racket.
"Tennis is just chess with balls....zugzwang"