Personally, I start an application because I want it to do something or what it does. If it has nothing to do it should sleep/wait by itself, without being babysitten. Otherwise, is a poorly written app and I’ll have to close it myself when done (if I can’t use a replacement for that app). The main problem is the memory used by that app. The kernel will have to compress-move and decompress-move memory as the app switches from active to inactive. The kernel might kill your foreground app because you like to collect suspended apps.
A web browser should take care of its tabs/windows, and give an option to its users to suspend inactive tabs (and maybe enable it by default), because the user doesn’t know what apps (javascript code) runs in each page, each time it is loaded and at each moment.
If an app consume 1% CPU when idle is one thing, 50% is another thing. I might accept the first case, but not the second. That app should be babysitted (not kept in brackground) or replaced.
I whish I had some kind of a notifications for the second case, e.g. when the cpu load is more than 80% for more than 5 seconds - make red the background of the top margin. The same with the memory consumption (bottom margin red, so I can swap and kill apps).