Do applications that remain in Recent Apps list take up resources? I always thought they remained memory residents but a developer told me otherwise.

Is there a canonical answer to this?
The answer can be found between the lines to the question What actually happens when you swipe an app out of the recent apps list?. There, eldarerathis quotes some trustworthy sources:


  [W]hat specifically happens when you swipe away a recent task is it:
  (1) kills any background or empty processes of the application (see
  http://developer.android.com/guide/topics/fundamentals/processes-and-threads.html#Lifecycle
  for what this means), and (2) uses the new
  http://developer.android.com/reference/android/app/Service.html#onTaskRemoved(android.content.Intent)
  API to tell any services of the application about the task being
  removed so it can do whatever it thinks is appropriate.


This could mean: if there are processes to be removed, they must be running -- which would be a "yes" to your question. On the other hand, if some app is listed in the recent apps list, that does not necessarily mean there was something left. "Recent" means explicitly what the word suggests: the user has recently used that app. Don't confuse that with Alt-Tab on your desktop computer, where you explicitly switch through running apps. The concept on Android is completely different.

So the final answer would be a "Nes" or "Yo". It could be such an app has still processes running. But even if, that's nothing to worry about: unless one of them is a service, those ressources are mostly limited to RAM and would be automatically freed as soon as they are required for something else. They don't eat your battery. Completely unused resources are, um, unused -- that is, useless. Why throw away something we might need shortly? The user could decide to switch back to that app. So if it's still loaded, it's not only faster available -- but it even requires less ressources (CPU, I/O) to activate it again, as it's already there.