Hahahahah!
I haven't said it was useless, but I understood that people used the whip to touch, and by that I understood as to "melee touch". And I think that the rules do not mention melee touch attack being made with anything else but hands. That's why I asked the question.
My question is about the reach. Reach weapons double the normal reach of creatures: Medium 5+5, Large 10+10 etc. But a whip is different, it has a stated reach of 15'. Just as the ranged weapons are, but a whip is no longer a ranged weapon! Assuming this includes the normal 5' of a medium creature, then it's 10' more, or a total of 3 times the normal reach. Would that be a better example to illustrate my enigma? A whip is longer than a creature is tall. A large whip (length = X) is to a large creature (ex.: frost giant = 15') what a medium whip is to a medium creature (human = 6'). Then why would a huge creature (Titan = 25') have a reach of 25' (15' natural + 10' flat for the whip) ?? It seems wrong.
So, would you give only 5' more reach per size category, the medium sized having a 5' freebie? Or would you give it 3x the normal reach (3x15' = 45') ? Or simply add 10' per size greater than small (med 15', large 20', huge 25') ?
While a 45' long reach would seem abnormal, we have to remember that the titan (actually a half-giant with an augmented expansion power for huge size) who wields it is 25' tall or more! And this same titan wields a 25' long greatsword, which should not be longer (or equal) than the reach of his whip (25' as stated earlier, 15' + 10').
This ruling isn't in the PHB, I guess, because it would reduce the whip's (and other reach weapons') usefulness to small characters... the same way that a small longspear has 10' reach, even tough it is about double the size of a halfling (6'?).