This type of rule would be used when you want increased lethality/autokill situations. If that's not the goal i'd suggest against it.
It would be slightly more deadly. Particularly for characters who have larger bonuses to Dex. You would still be getting a save just no bonus for Dex to that save. For a fighter or other melee type using Plate Armor and Heavy shield chances are pretty good he'd lose little if anything from his save given that his Dex bonus might be +1 on a good day. Your typical Rogue on the other hand might be a bit SOL. (And I can see a slippery slope towards denying things like Evasion).
Heh - I don't think even paralyzed foes get a penalty to Reflex saves... but if you're shaken or sickened you do.
paralyzed creatures are helpless... a helpless character has a dex of 0, so their dexterity modifier is -5. Thus a paralyzed character at best has a much worse chance of making a reflex save then when he is not paralyzed.
This has bugged me too.
Can you think of any unexpected consequences of hourseruling that when you have lost your dex bonus to ac, you also lose your dex bonus to reflex saves?
Yes, I can.
Right now the game rules are fairly balanced. If you want to nuke someone with a fireball, that target gets a chance for it to fail (or only partially succeed).
Change the rules so that there are easy ways to rob him of his defenses, and suddenly a Stun/Fireball combo (or any other "lose DEX"/"REF Save" combo0 becomes significantly more deadly.
Your players will exploit that. Then you'll have to exploit it with monsters and bad guys to stay even in the "arms race". Pretty soon everyone will be doing these combos because other tactics are inferior.
Breaking balance can have far-reaching consequences. I've made that mistake, and exploited the mistakes of other DMs, far too often.