The main advantage of having the psu in the bottom is -at least in my opinion- the so called 'weight point' of the case. The lower this theoretic point is (= main portion of the weight is located lower) the more stable the case is. Assumed that the PSU makes 20% of the cases' weight it could gain you some serious stability.
Secondly, the psu does not take hot air in its intake when it's located at the bottom. This makes it quieter
Thirdly, when you more the psu from botto mto top you can place another fan/larger rad at the top of your case.
Fourthly, the psu fan often 'bounces' air back because of the high air resistance (inner components) inside the psu. This causes the warm air not to be exhausted properly.