1. It really doesn't happen very much
2. Someone is usually willing to trade if you ask nicely and say "I was auto-filled"
3. No auto-fill would inevitably mean longer queue times
Besides, isn't it kind of healthy to play all 5 roles every now and then? If there are legitimately only 2 roles you can play without doing horribly, you're putting a limit on how high you can climb. You won't understand the perspectives of the other 3 positions, and will therefore inhibit your ability to be an effective teammate. One-tricking is fine and dandy, but I think it's important to be at least half-decent with a champ or two in all 5 positions.
If you're genuinely not in favor of Auto-fill, I'd love to hear your perspective in the comments so I can see where you're coming from.