Even as part of the minority that use window managers on linux I find the idea of merely using the window manager exclusively in place of other forms of UI for multiple documents/pages/buffers within a gui app as fantastically underwhelming.
Lets look at firefox as an example. The fact that a vertical tabbar is more useful in this case due to screen shape and size and it being useful to be able to read a bit more of the text when you have a lot of tabs. This could in theory by done by your wm even though I've not seen one that does. Further tree style tabs shows a visual hierarchy of sorts that can't be replicated by your wm because it doesn't know what link was opened from what. Further you can perform operations on groups of tabs, like closing them, bookmarking them, reloading them. Its difficult to see how your wm could infer this hierarchy and how it could allow application specific operations like bookmarking even if it could. Further its useful to sometimes open a number of links in the background before switching to any of the above. This is trivially effected within the browser but challenging outside of it because the action of opening a link happens inside the browser.
Within my irc client various channels are arranged according to network and color coded according to activity. Right clicking on individual channels or networks allows specific actions regarding those networks or channels. None of the above would work with a bunch of stacked/tabbed windows.
I could make a list for emacs but the complaint is the same wm ends up being an unfixably poorer alternative to dealing with multiple documents in the app.
I use i3wm not because its awesome at managing multiple documents but because its simple and straightforward and 90% of workspaces, which span only a single monitor, have only 1-2 windows on them. Rarely 3 or 4 and never more.
Window managers are just unfixably mediocre in this respect because the UI doesn't have access to context and can't differ based on use.