I don't really see how what you said makes Seviper bad, sure he doesn't help Toxapex much, but that's just because Toxapex is to slow, but if any cards that provide poison come out then seviper could come in handy. Sure, right now he isn't likely to see much play, but he could see play in the future, he at least has some amount of potential. And in expanded I'm sure he could see at least a little play as maybe a one-of, being used in conjunction with Hypno-toxic laser.
By taking valuable bench space that we can't afford to lose post-rotation? 10 damage is pretty much obsolete if you're resorting to using poison to do damage. It's getting hyped for no reason like Toxapex was, since it will never see play throughout its' lifetime. And no one can argue that it will see expanded play because of Virbank, Laser, and Seismitoad, because it won't. The primary problem with Seviper is that it takes benchspace that standard can't afford to waste, and expanded is too fast paced plus, uh, Keldeo.