That's kind of the trade off. It snares targets for a fairly ridiculous amount of time, but it's also fairly easy to dodge.
I should note that champions in LoL aren't exactly mobile to begin with though - Q may have turtle speed, but your targets aren't exactly cheetahs or road-runners.
I mean, Mario makes Kassadin look immobile. Heck, the avatar of most games that's simply capable of spriting and/or jumping would have a solid edge mobility wise vs. the typical LoL champion.
When you're playing a typical marksman or support vs. Morganna you pretty much have to dodge Binding at a 90 degree angle.
With slow skillshots you can also take advantage of the terrain, positioning, and movement of opponents to give them a very high chance of landing. If an opponent hugs a wall, you can fire a skillshot so it will barely connect with anyone hugging the wall - preventing moving into the wall from dodging it, which makes the skillshot's radius almost 3 times as wide, seeing as they have to move into the skillshot and through the entire diameter rather than just dodging away by it's radius.
You can also set up skillshots to be a catch 22 deal for your opponent. Like, run in the narrow hallway away from my team and get binded, or run into my team and run into my team.
Forcing the choice between a lesser of 2 evils is a solid way to go about it, putting the binding where they want to be rather than where they are.