> How much useful extra space do you really get? You can stack separate washers and dryers on top of each other without issues.

Stacking still takes up usable space; in rooms designed for unstacked washers and/or dryers the space above is frequently used for cabinets

Enough space for me, but you are right stacking would provide similar benefits. Don’t undervalue not needing to transfer clothes to your drier, and you never have to worry about mold and mildew in your washer.