The idea would be that charging Xerath's Q would give it percentage Magic Penetration starting from zero, as well as range.
Right now, when you're laning against Xerath on either the PBE servers or the Live servers, and he roots himself (Live server) or starts charging his Q (PBE), you have two options. The first option is to abandon your farming and back off until the charge-time/root-time ends, which is no fun and is often your only option against a fed Xerath. The other option is to try to dodge sideways, which usually doesn't work. If you attempt to engage on Live Xerath he uses the speed boost from his W to get away easily after hitting you for some damage. If you attempt to engage on PBE Xerath, he launches his Q immediately, does full damage, and runs away, using his W and/or E if you continue to chase.
It's frustrating, and doesn't require much thinking on Xerath's part.
If the damage on PBE Xerath's Q increased as he charged it, engaging on him would be much more viable and interesting. The longer Xerath waits, the more damage he deals, so he is rewarded for cutting it close, rewarded for taking risks, rewarded for giving a bigger opening for counter-play.