E is a ground targeted ability. If you think it is best to aim the center on an enemy champion's current location, go ahead, but Riot isn't going to change it to a single target ability. Obviously, the most useful thing about Veigar's E is when it stuns an enemy champion so that you and other allies can most easily land harass. I think one of the undervalued ways that Veigar's E can be used is as a zoning ability. You can easily scare your enemy into backing off or force them to move toward you with a well placed E and making the ability automatically center the champion gives them an option of which way to go . If you place the whole circle behind your enemy, it's unlikely they will chose to retreat that way and risk getting stunned.