Any trick you do to reduce resource consumption from spaces (such as suspending and evicting) can also be applied to normal windows. Even the currently shown page can be evicted if your browser is hidden, should one want to do that.

However, listing this as the only difference when asked make it sound like you're selling "spaces" as only a slight performance improvement and nothing else.

Can you elaborate more on this? My understanding is that on modern browsers, most of the memory usage comes from the tab's contents (pages), or extensions. In both cases they don't scale up with the number of windows.