Sorry but thats not truth, i've always use my routers without top case and im just gonna explain you why.
All my routers, 2x WL-500GP, 2x WL-500GPv2, 1x WL-500W after 1-2 years of working duty popped capacitors inside because high temperatures, if they were designed to stand it this wouldn't never happen. The 2 models with top case opened (1x WL-500GP, 1x WL-500GPv2) were about 15-20 degrees less compared with the same ones closed and they are still working nowadays. Temperature is always a concerning factor on electronics even the if the components are very high quality and could handle it. Its always better having lower temperatures no matter what.
Believe me i have alot of experience on this on the last 10 years
Well i have 15 years of experience with routers and i have never had a router to fail due to heat! But what you are discribing is : wl-500 routers arent any good with handling heat!! I have used several brands. 5 different dlinks, 1 netgear, 1 zyxel, 1 dovado, 1 asus, non of them has failed due to heat and some of them has got quite hot. So does the fact that WL-500 cant handle heat meen that Asus RT-N66U will go down the same road to death? This is my first Asus router. So are the Asus bad at designing routers? Or what are you saying? I know that heat is a killer but what im saying is that routers are normally designed to handle heat !?