irdeggman said:
Actually per the DMG they are not the same weapon.
It requires EWP (katana) to use a katana in one hand - it doesn't say that EWP (bastard sword) works for both - even though it says it is functionally a bastard sword. Although technically they are really nothing alike as far as use goes.
Well, stat-wise, they are identical. The only difference is that a katana is always masterwork, and it costs a little bit more. In addition, when the DMG refers to it, it's more or less assuming that the katana replaces the bastard sword entirely in an Asian campaign, thus making the bastard sword proficiency feat unnecessary, since the bastard sword is a mainly European weapon.
As such, in a multicultural adventure (Complete Warrior Samurai, for instance), it would make sense that being proficient with a bastard sword would mean being proficient with the katana, and vice-versa. Otherwise, if the DM specified that a monster's bastard sword was in fact a katana, boom, the party Fighter's been thrown out of the lootpile.
As for realisticness in terms of weapon use, true, they are different. But, I think that the reason why the bastard sword is exotic is the weight and length of the weapon - it's initially too big and heavy to use well. The same is true for the katana. Using them two-handed really just requires being decent with a sword (the overall martial weapon proficiencies given to most full-BAB classes), the real exotic bit is the weight. So, since the bastard sword and the katana weigh the same (and, thus, we can assume that they are close in length), can it not be assumed that if you have the single-handed strength training required for a bastard sword, that you could manage the same with the katana?
Regardless of personal opinion, though, this seems like a really obscure scenario that would have to be resolved by a DM judgement.