My thoughts:
 
Sometimes it takes a lot of work to get just a small return.
That small return could be worth it.
 
Maybe it's time to scale back...that's okay because any contribution, no matter how small, is valuable.
 
I would rather contribute this way, where I have an idea of what my contribution is doing, as opposed to just giving money to some organization and not really knowing how it got used.
But sometimes I think "is all this worth it?" too.
 
The bottom line for me is that I feel good running DC projects, like I'm doing something that is not as simple as just giving money away and it's going toward things I care about: medical research, physics, math, astronomy, etc.  Also, I love tinkering with PC stuff.  It's like running my car on the dyno or at the race track to see what it can do.  Most people don't even come close to utilizing the full power of thier PC's or cars.
 
Some cool things I like to check on as a "return" on my contribution are the galaxy maps from pogs.  I can see right in my dashboard the stars and planets I helped map.
 
Plus this community is about as good as they come, and interacting with you guys adds immensely to my motivation.
 
Finally, we are all volunteers.  So whatever you end up deciding...is the right decision.

I personally would rather hand someone food rather than hand them cash to be spoiled where not intended. DC'ing lets me be involved even on a non-personal level that you don't get with handing someone money. Money donated to charities/non-profits aren't always wisely managed or even divvied where intended. A good chunk goes to salaries (and let's face it that is profiteering) and other replicated services. Why do we need a thousand cancer charities? Not all of them are finding cures. Not all of them really contribute a large chunk to research. What does contributing to research even mean to a non-profit? Is it buying their gear? Paying their salaries? Giving them access to resources? Or advertising? What about lobbying power? Do you agree with your money paying for all of those? I myself like more control on where my "charity" goes. I want to see it doing what I personally want it to do. DC'ing does that. I know my CPU resources are crunching research XYZ. If it gets used, then that is great. If it doesn't, then it is no worse off than the research my $$ donation would have supported as it has the same risk. But at least I know my contribution went to where I wanted it. Throwing millions of $$$ at a problem isn't necessarily going to fix it any quicker. It just means more waste or more profit occurs. The rest is variable.
 
Why do I DC?
1. I have the hardware already sitting around and have the know how to use it for potentially bettering the world. I actually sought out DC before even knowing it existed.
2. I've experienced horrible disease effects like most people and get the squishy warm feeling that I did something rather than just drop change in a can.
3. I like the community, but let's face it.. you can get that anywhere. But here I can join the sense of doing good with those that are a bit more techie. It has helped me understand hardware/software well beyond what I would have bothered learning if I had not.
4. Though most projects are poorly ran and do not track results beyond the publishing of their papers, at least WCG has done really well with keeping up. I feel you when you say you wonder if any real results have occurred but in just 5 years? You gotta be more realistic. A new drug could take multiple decades to make it to market and that is typically only after multiple candidates were weeded out. Just knowing WCG has gotten drug candidates that are already into testing is huge. The fact we know our contribution accomplished that is huge. That is more that what you will ever get writing a check to XYZ organization.

What an awesome question.
 
Here is my take...
 
I have written $10k checks. Feels AWESOME at the time. Not long after, it is all forgotten. And the next wave of "Please donate money so we can do XYZ" comes along. I get excited to help build a hospital in Haiti, using their own labor and train them the skills at the same time! AWESOME!. Only to have an earthquake or hurricane wipe it out. Now what? We have to start completely over!
 
So what I have found to help my inner need to feel I am giving back to humanity is to do both now. From helping buy musical instruments for kids with needs, to raising funds for a woman's shelter to just simply giving money to a school to pay for all "Ladies" needs and sports exams for those in need.
 
And then for the long term effect. I Crunch and or FOLD. Even if my life ended tomorrow, I know for a fact that what ever energy my computers have spent, will go to helping humanity forever. Even if it is a tiny little spec vs. the need. No storm can take that away from us.
 
So that is my take on it. I do both. One that my community has immediate needs for, the other is for all of humanity. All I hope is when I am gone, I am remembered in a fond manner with everyone knowing my heart was always meant for the greater good.