Rather than a concern, its more that it allows you more flexibility to include more art if you want. For example, if you're using the same tiles for battles and overworld anyway, it doesn't make make much sense to change layouts.
If you wanted a lot of different tilesets/environments, you could do a low resolution overworld tileset that includes a lot of variety such as forest, snow, desert, water, ect. and then a more detailed tileset for each of those zones for a zoomed in map that you would load in layouts as needed.
The other example was characters - if you wanted high resolution art or heavily animated enemies ect, it would make sense to load them as needed in a separate layout rather than have all your enemies loaded in memory all the time, limiting your memory use for other things.
If you don't expect to run into memory issues, then you probably could stick with just leaving everything on one layout =) Just wanting to chime in as it is pretty normal for JRPGs to load into and out of battles or changing zones, which would be the equivalent of changing layouts for us. It is usually because all the art they want to use in the entire game can't be loaded into memory all at the same time.