In my opinion, here are some major problems with one "all-charity"(excluding corruption):
Order of Importance:
Who is to decide whether AIDs is more important to work on first, or cancer?
Case A: Deciding through personal matters("irrational").
Case B: Deciding through death-tolls("rational").
Example of Case A: Let us assume that the person deciding which problem to be worked on first has a wife diagnosed with cancer. Assuming he is a selfish human being, he will choose cancer as their first target. Even in a council, most people will be biased in one direction or another due to life experiences (friends, relatives, severity of disease in homeland, etc).
Example of Case B: Let us assume that AIDs has twice as many people in the year 2010 than cancer. And thus, let us assume that, despite his wife, the aforementioned person in charge decides to work towards AIDs. What about all the people who have lost people to cancer, or are currently afflicted with cancer? Do their contributions matter help others with cancer (assuming they contributed in the support of cancer research).
This leads to a second major problem:
Donations
Humans are generally selfish beings. That having been said, how many people with cancer-diagnosed relatives are going to donate to an association that won't focus on cancer, but may instead focus on autistic youth? After all, they don't have an autistic child, why should they care? However, keep in mind this problem is based off of the assumption that the would-be donators are selfish.
The benefit with single-cause charities is that the donators will know what they exactly they are helping, and so many people will be content with donating to their own interests. Wouldn't it throw people off from donating if they know that their concern isn't being specifically worked with?
Knowing when to stop
ShaggyJebus wrote...
or it is deemed impossible for the time being
Exactly when is this finite line drawn? For example, which would be deemed more "impossible" than the other? AIDS or Cancer? Autism or helping the homeless(you can't get all of them)? Stopping child abuse or helping war-torn countries? All of the above cases have been worked on for a while, but none of the them have seen a definite solution. Is this where we draw the line? Or do we draw the line after several more years, thousands more failed attempts, millions in money being spent attempting to fix these problems?
When this line is drawn, and the focus towards the cause is cut, what about those who donated to this organization specifically for that cause? Wouldn't they feel betrayed?
I would prefer the millions of charities that we have at the moment, for the reasons stated above.