The current system, where UN peacekeepers are taken from many different national forces, is not terribly effective. How can these forces work together when there are not common policies, common equipment, a common command structure? Also, armies from nation-states may have their own agendas, whereas a UN force would be more neutral.
It seems to me that the UN should really focus more on what I think they should be doing and that is humanitarian aid. I think that they should focus more on that aspect than having some kind of standing army. I can understand some people possible wanted the offensive power though.