You only need a feat if you want to attack with multiple weapons in the same round. Nothing stops you from having a Rapier in one hand and a whip in the other, and attacking with the whip when you're 10 ft away, and the rapier when you're right next to your opponent.
Sure, but now you've either given up the character concept of weapon in one hand, whip in the other (because presumably you're swapping the whip for something else when you're within 5') or you're seriously gimping yourself by holding a useless item in the offhand.
The whip's advantage is that it is a Finesse, Reach weapon. It gives up a couple of points of damage for being able to attack from out of normal melee range, such as from behind your tougher characters. If you're not making use of its primary advantage, it makes sense to use a weapon that doesn't have it if you want the minimal damage increase.
First, Finesse itself isn't an advantage. It's a mutually exclusive property with Versatile, and it doesn't actually grant any advantages (Except maybe that Rogues can use them for Sneak Attack.) And range is only an advantage very, very rarely. So, yeah, if you're proficient with martial weapons then maybe it makes sense to carry one on your belt for those edge cases, because they're lighter than Halberds.
But otherwise it's a pretty terrible weapon. You're giving up 2 points of damage per attack when compared to the alternatives. That's equivalent to the Dueling fighting style.
There is a specific feat designed to grant the simultaneous use of non-light weapons as dual-wielding. I don't think that being able to pick the best advantages out of other feats and combining them is a good way to balance new ones.
It shouldn't be "competing with dual wielder" because it shouldn't be granting the same effect at all. As with all feats, you should get benefits for taking both if you wanted.
TOTALLY agree. But that doesn't mean it isn't competing. Great Weapon Master and Sentinel compete with each other, in the sense that for a lot of builds they are both great feats and with one ASI you only get to pick one, but they are also great together.
Now, I'm not sure exactly how to make the same thing work with Whip Master, but that's my goal. Again, if Whips were simply "light" it would solve most of the problem.
I'm not seeing that happening. Exchanging a couple of points of damage for Reach isn't being punished.
Well, as noted above, I think we just disagree about that. How many whips are you actually seeing used as primary weapons? I've yet to see a single instance of it.
If you make it automatic as your previous suggested feat (did you mean Acrobatics or Athletics?) then as you have written it, it would be expected to apply to every situation you were making that check, whether you were making the check to reduce your falling damage or to wriggle out of a grapple.
Er, yes, I agree that's a problem. That's one reason I'm posting here...to see if anybody has any solutions to how to write it such that it's simple, fun, and useful, but not overly broad.
Doesn't that "logic" apply to every weapon in the book that isn't already a monk weapon though?
Why is "logic" in quotes? Just snark?
Generally the more damaging weapons (pretty much anything in the martial list) is not a Monk Weapon because their damage is 1d8 or more. Part of the balance of the monk class is that they do less damage at lower levels and make up for it with multiple attacks. Then their base damage scales as they increase levels to keep pace with the damage boosts of other classes (scaling Sneak Attack, multiple Divine Smites, etc.). Most of the weapons that aren't currently Monk Weapons would do too much damage at lower levels, and mess up that balance. Since the whip only does 1d4 it wouldn't really affect balance if monks could use it.
So, no, the same "logic" doesn't apply to every weapon.
