Not every one is accustomed to every feature of the engine and they need not be. Anyone can use whichever method which is easier to them or more comfortable to work with. Using rooms is easy for most people. You just import the background and do the necessary scripting.
Moreover, because of how things work(for AGS at least), room backgrounds compress much better than sprites, so it actually reduces the game's size if you use rooms instead of importing a number of sprites. IMO this is a big advantage.
Moreover, because of how things work(for AGS at least), room backgrounds compress much better than sprites, so it actually reduces the game's size if you use rooms instead of importing a number of sprites. IMO this is a big advantage.