@Mor - Although you make good points, I think it's important to note that there is an even more important reason that microfinance funds should be directed specifically towards women. In general, they are simply not going to be given many chances, and not nearly as many as men. If the funds are not put aside specifically for women, then it's not a matter of a 50/50 split.
Many impoverished societies still have entrenched patriarchal systems and will automatically give any resources to men because they are considered to be the leaders by default.
Basically, even if women weren't better at handling the money, they would still need organisations to earmark funds for them so that they could get a fair chance to improve their lives in this kind of system. The fact that they are often better placed to improve the lives of whole families is wonderful, but it's not the whole story.