Dollars are fungible; but how you react to that fungibility can have real, and long term, consequences that you can't necessarily cheaply or quickly buy your way back out of.
In the case of South Africa, say, you've got high crime rates and substantial pockets of poverty; but you also have areas of fairly well developed civil society, economic development, higher education, and similar. Unless you are god's own gift to social engineering, do you really want to bet that you can divert resources from the less-dysfunctional areas of the country efficiently enough that you can fix the defective ones before the functional ones brain-drain away to somewhere with higher salaries and lower murder rates?
One must, of course, do something about the festering issues (even if you have no humanitarian interest, crime and low quality of life are very, very, expensive in terms of guard labor, instability, etc.) and one must also be very careful not to treat a given sector of the country as 'well developed' just because it's rich and looks good in a suit (any Russian oligarch, middle eastern petro-sheik, or American white-collar criminal could say that much, and those tend to be cancers on their respective societies); but unless you have the good fortune of having social problems that can be solved with mere money (rather than money along with sustained good governance, anti-corruption efforts, and assorted other tricky bits), you probably don't want to slash support for the parts of your society that aren't totally screwed merely to hire more of the police that aren't keeping crime in check now to wander around.