Why is Central Admin on an application server by default? It seems like if you wanted your app server to truly be an app server, then you wouldn't have it serving any sites. This seems like it could also be a problem for availability. If your app server goes down, then so does Central Admin. Of course this is assuming that there is only 1 app server, but most set ups I've seen, or read questions on, have 1 app server and 2 WFE's. Why not have Central admin on the WFE's?
I'm not sure on Microsoft's reasoning, but when I design farms, I purposely leave CA on an app server as it is not a high traffic site and won't affect performance of the app server.
It's the management tool for your farm, so keeping it on servers that its managing makes sense to me. It also allows you to extract a level of security by not putting it on a web front end, which could be accessed via a load balancer, instead it's on server that's behind the scenes that less users would even know about.
Just my 2 cents.