I've had to think quite a bit about this too, as my husband and I are in a somewhat similar situation with his parents. They did pay for his University education, which we are immensely grateful for. However, they are unbelievable financially irresponsible, and have no retirement savings whatsoever. They have a fairly high income, but they spend it faster than it comes in, so it doesn't really matter. And the amount of financial support they have given us pales in comparison to what they've given to my husband's siblings, who are just as irresponsible as they are. In short, we are the most financially stable people in the family, and those who "ought" to have the most obligation to take care of them when they can no longer work and realize they have no assets, will not be able to.
Helping them plan for this eventuality is utterly hopeless - they will not accept advice nor entertain even the slightest drop in standards of living.
It really worried me for a while, but I have come to the same conclusion obviously many others here have - we will shelter and feed them if necessary, but I would not feel comfortable giving them money. And while they are not terrible people, they have really not been supportive in any sense other than financial, while my husband has been working like crazy to keep their business afloat.. so I guess I don't feel like we have that "good parenting" obligation, and in my admittedly biased opinion, I feel like we have already paid back the financial aid many times over.
If it were my parents, I would feel differently. They are amazing people, financially responsible, and would not need to rely on us for that sort of thing unless some disaster outside of their control caused it. If I handed them a cheque, I know they would use it wisely. But I don't see the point in giving cold hard cash to someone who will blow through it just like they did their own.