Two stats per defense!

[Henry]

Well, we could check the text for the Insightful Reflexes feat from Complete Adventurer, since that allows you to use your Int modifier for Reflex saves:

"Your keen intellect allows you an uncanny knack for evading dangerous effects."

Okay, who wrote that crap? That's terrible.

Anyway, here's the explanation. Someone who relies on their Int for Reflex defence is all about reading situations and anticipation. You deduce what the enemy is going to try next, so you're ready for them when they do it. Someone who relies on their Dex is all about reaction. You don't anticipate, but you react with great speed. It's really quite simple.

How about your stereotypical brain in a jar? If he's a superintelligent brain, he's got a 30 INT or so, but a 0 dex? How's he getting out of the way of the fireball? Telekinesis?

 

[Fifth Element]

How about your stereotypical brain in a jar? If he's a superintelligent brain, he's got a 30 INT or so, but a 0 dex? How's he getting out of the way of the fireball? Telekinesis?

That assumes there's no penalties for being immobilized, which a brain in a jar would effectively be. Or no penalties for having 0 in an ability score.

But if you have to go to that extreme for an exception, the rule works fine.

 

[Simon Marks]

If he's immobile, he's helpless. Ref = 0.

As true in all editions of D&D as anything else.