I don't think that they need to be coding everything in the application. But they should be given some tasks. Some "architects" are basically hacks with no technical experience who pass themselves off as "legendary coders" and design systems that add weeks or months to the amount of work you would have had to do if you didn't have an architecture. If they can't write code they have no business being in that role...Because the idea is an architecture should make the application easier to develop and maintain. But if the architect cannot develop, how would he or she know how to make an architecture?
Also, even the best architects make bad decisions. Sometimes something looks great on the white board but then in reality an architectural decision ends up making things a lot harder than they should be. If the architect has to eat his/her own dogfood (and use the architecture) they are more inclined to want to correct bad decisions or to architect ways around them. Also by touching the code they are at least a little familiar with it. So if a developer comes to them with a coding issue, the architect's eyes won't glaze over as he/she dismisses the developer and says the developer is doing it wrong but offers no insight onto the right way to do it.
The architect doesn't need to be doing big projects within the code base. But they should at least be doing small tasks within the code base that force them to use their own architecture. Also the small tasks will probably involve reading a lot of other people's code to give them a sense for how people are using the architecture and whether anything can be done to improve it.
