now you can place them in the middle of a road or a field of nothing and then when a single unit walks over it it is dead for you had no idea it was there unless you had a Minesweeper close.
Guess what happens when you run into a mine?
Unless you have a minesweeper nearby, your unit just got wtf-slapped. Think of demo's as much bigger, nastier mines. Consider bringing a minesweeper or two along with you whenever you face Commandos if they pose such a big problem - But I'd recommend keeping minesweepers on hand anyhow as it's pretty sensible, just like keeping a Jeep/Bike or two around.