People advocate it because other people advocate it. Around gym circles, the plural of anecdote is fact (props to somebody at Avant for that line). If there were any appreciable benefit of it that outweighed the unappealing negatives associated with sharply elevated glucose and insulin (ignoring the fact that amino acids can spike insulin) then people would be able to give you a good reason why you need to mainline high GI sugar into your bloodstream post workout. No one will ever sincerely tell you that they know for a definitive fact that their workouts and results were outstandingly and markedly enhanced because they took a huge dose of high GI sugar post workout instead of something healthy. People act like their workouts went through the roof because they imbibed some sugar that's a 4000 on the GI as opposed to eating PWO what they advocate eating all day long. It's not like you even need that much sugar PWO anyway.
From a product maker's standpoint, maltodextrin rules because you can pepper your food with it and call it "sugar free," meanwhile people's legs are falling off from diabetes because they ate your lyin' ass food all day long.