I just wanted to ask this for clarity: does adding the feycraft template (Dungeon Master's Guide II, p 275) to a whip or a rapier allow you to use your dexterity modifier in place of your strength modifier to attack rolls with it without the Weapon Finesse feat?
Rules-as-written, yes, it’s pointless. The feycraft template only accounts for the light/one-handed/two-handed category of the weapon, with no exception made for exceptional weapons like the rapier or whip.
However, I have generally found that DMs (myself included) are willing to make their own exception for these weapons, allowing them to function the same as light weapons ordinarily do. Basically, most DMs I have played with have interpreted the feycraft template as reducing the weapon’s category for the purposes of finesse, from unable to able, and from able to automatic even without Weapon Finesse. Since these weapons are already able, they should move down to automatic. These DMs often also apply this benefit to the spiked chain and elven courtblade, two-handed weapons that are already in the “able” category.
So my suggestion is to ask your DM.
A feycraft rapier or whip is usually not a good idea
The Feycraft Template (Dungeon Master's Guide II 274-5) on Feycraft Weapons says, in part, that a 1-handed weapon with the template
can be treated as a light weapon for the purpose of the Weapon Finesse feat, but not for any other purpose. If the weapon would normally be considered light, the wielder can apply his Dexterity modifier instead of his Strength modifier to attack rolls made with it, as if he had the Weapon Finesse feat. (275)
Both the rapier (PH 116, 120) (20 gp; 2 lbs.), a martial melee weapon, and the whip (PH 117, 121-2) (1 gp; 2 lbs.), an exotic melee weapon, are one-handed weapons with special exceptions that allow them to be used already with the feat Weapon Finesse, so adding the template feycraft to either weapon reduces the weapon's damage but provides no other effect.
There are probably (ahem) edge cases wherein the template would be useful on such weapons anyway, but I can't think of any outside, like, y'know, role-playing.