What Gates is doing is brilliant by long term planning. By promoting healthcare, education, sanitation, clean water & information technology in the developing world he is helping to build their economies and scientific base of the world. A world of healthy, financially secure, literate people is very important for everyone.
In the 1970s over 60% of Chinese people lived on less than $1/day. Now China, due to record economic growth is becoming a major research hub of earth. China does tons of research that is going to be and already is important to western life including research on alternative energy and an AIDS vaccine. Gates is trying to do the same thing, he is trying to increase the standard of living & economies of poor countries which is not only humane but increases the talent pool that humanity can call upon. A world where everyone is literate, financially secure and healthy is better than one where 15% of the world is and 85% arent. By doing so he is improving all of humanity as the 21st century will be a century of ideas. I think every dollar spent on developing world infrastructure provides about $7 in economic growth.
Point being, Gates is increasing the scientific base of the world. Perhaps some impovrished kid from Africa will grow up to be a theoretical physicist in 2030 if his country has good infrastructure. If his country has shitty infrastructure, no education, no healthcare, no roads, etc he could die in childhood or grow up to be a frustrated & bored farmer.
The US already spends about $300 billion a year on scientific R&D and the world spends close to a trillion on scientific R&D annually, Gates throwing another 3-5 billion a year at it wont matter much.
Research has been done into how to combat and avoid corruption. Gates isn't going to throw money at Mugabe's government, all the info I've read on him says he is very hands on and results oriented.