her Q is on a 3 second cooldown, and isn't THAT hard to hit if someone is standing there hitting you
also she has 2 other spells that re-apply poison if you can't wait for Q to be up, not to mention that laning with a singed or teemo makes it that much easier
honestly I think keeping poison is easier than keeping up urgot's targeter
edit: AND poison does DoT damage, which urgot's targeter doesn't do
his ONLY advantage is range. that's it