Ten players on each team line up 5 yards apart for an onside kick that still has to go 10 yards. The return man can line up anywhere behind those 10 players that he wants. So it's a game of chess with the kicker for that position, while the other 10 on the receiving have to scramble back to get to the ball before the kicking team, which can't get a running start. The receiving team can touch the ball before it goes 10 yards, but like it does now, it becomes a live ball.
Lots of strategy for the kicker. Do you pooch kick it over the line and away from where you think the returner is going to be? Do you try to bounce it off a player only 5 yards away, so your team can jump on it? Plus, since you don't have to change the alignment, the onside kick could still be a surprise.