It didn't look like everyone keying in on Giovinco slowed him down at all! Given that Giovinco only needs about 2 feet of space to work his magic it's hard to see how even having two defenders man-mark him would do opposition teams any benefit, in fact it would ruin their defensive shape (giving opportunities to Bradley and Altidore) while Seba would still score goals!
Think about it, Vanney has created a winning formula and you want to change it?