- Wizard that has no self-defense skills and is physically frail: low Con, like Raistilin. Now, if that same Wizard cast "Protection from Poison" (kust made up), that'd bump his Con saves, but not his hp. If saves and checks were the same thing, the spell would boost his Con, which would have the side effect of boosting his hp.
Con penalty or not, the wizard will still gain hit points with each level. A 20th-level wizard with Constitution 8 has more than four times the hit points of the toughest 1st-level fighter. And I would assume "Protection from Poison" only provides benefits against poison specifically; if you give a general boost to Con saves, you're getting protection from poison, disease, death spells, and anything else that calls for a Con save.
- Juggler who can't pick a pocket: Good Dex, but no Sleight of Hand proficiency.
Sleight of Hand is the skill that's used for both. Not having Sleight of Hand proficiency hurts you just as much with juggling as it does with picking pockets.
Or even better, Sleight of Hand proficiency being used with Charisma (for showmanship), but poor Dex (the proficiency offsets the lack of natural coordination).
- Blind character with good hearing: good Wisdom and/or proficiency in Perception, but automatically fails Perception checks based on sight.
- Nearsighted: character has disadvantage on Perception checks relying on sight beyond what he can hold, unless he is wearing spectacles.
You just made up a bunch of new mechanics to handle these situations. Do you propose to put those mechanics in the rulebook? I would hope not--it would be ridiculous to jam all these things in, when the vast majority of PCs won't ever need them.
What I'm getting at is that just because you
can write distinctions like these into the rules doesn't mean it's worth the bother. If very few people are interested in making characters who are clumsy but have excellent manual dexterity, we don't need rules for it in the book. On the rare occasion that somebody's got his or her heart set on such a concept, the DM can homebrew a solution.
Similarly, I see very little point in having rules to separate out Dex saves from all the other things you use Dex for. Sure, you can in theory imagine a character concept which involves being generally clumsy (poor unarmored AC, poor Stealth and Acrobatics checks, low initiative, et cetera) but skilled at avoiding area effects and rays (good Dex saves). But in practice, how often is anybody going to hit on that concept when thinking up a character?