Good guacamole has fresh onion in it. When I tried to make guac yesterday, the only onion in the kitchen was starting to go bad, so I needed a substitute. Scallions lend a much, much stronger flavor to the guac than onions.
But it worked out well. We like the guacamole and it’s got a nice pungent flavor. Scallions are more expensive than onions but when that’s what you’ve got on hand, that’s what you use.