Well, there are some flaws in your argument: Gates could not help in any way the AIDS or malaria sufferers from his CEO position (as Microsoft is powerless in the pharmaceutical industry and has a rather slight footprint in Africa), nor Buffet from his multiple investments has anything to do with pharmaceuticals and very little with Africa.

Having all their employees happy, environmental and paying more taxes will not help much the AIDS or malaria sufferers, as the world status quo before their existence proved.

I tend to look favourably at their philantropic approach as being a foundation with independent and stable income allows them to approach problems that nobody has approached before. As always, you don't have a guarantee that they do it well, but government or WHO existed before them and have not been very succesful in promoting malaria research...

And it's better than spending their disposable income in militias or luxurious life.