moritheil said:1. Fair enough.
2. That's not exactly precise. What I said was more along the lines of, "I can think of a very plausible reason that an ability penalty is a very different thing from ability damage," in response to your question about the need for differentiation. I gave a reason why things could be immune to poisons, but not bestow curse, as opposed to simply assuming it for unspecified reasons. Evidently, you do not agree that the reasoning is plausible enough to sway your opinion. I can respect that.
3. As a separate but related issue, by the rules, the two are different things. I happen to think that this is entirely plausible (see #2), but for anyone who doesn't, I'm sure it's a very easily explained house rule to consider them the same thing.
4. I can extend my rationalization to cover any example you present, as I have shown, so I'm still going to take issue with your calling it "irrelevant," because you are essentially stating that there is no rationalization at all for separating the two categories.
Out of order:
4. Would you be more satisfied if I just called it a non-sequiter instead of irrelevant?

2. Well I completely agree that things can be immune to poisons but not bestow curse. Extending it further, I can completely see why a theoretical creature could be subject to a penalty of a sort, but not damage of that same sort.
Honestly I think what bothers me most is that it seems to be just as much a rule of omission as a rule by intent; the rules are a bit lacking in the ability penalty area. What happens if an ability penalty takes an ability into the negatives ? What happens if you apply bestow greater curse to a construct to give it a con of 1 ? Most situations (such as those; they aren't very good examples) can be dealt with with just common sense; but then you start to run into the ones where people differ on what common sense is. Thats what the DM is for, I guess, but it feels a bit sloppy.
Mind you that feeling is probably mostly compounded by all the stuff from FAQs, erratas, splat books etc that doesn't make sense or flat contradicts other rules. Heck even the core rules aren't immune; whether racial bonuses stack or not depends on whether you believe the DMG or the PHB, and whether you think the primary source just had an editing error or not

And like I said above; when designing spells it seems like the sole measure of whether the spell does damage or inflicts a penalty is whether they want it to stick around after the spell wears off or not. Having creature immunities kicking in based solely on that factor seems a bit off.