People ask for this all the time but it's a really bad idea.
A.) This works out to millions of extra dodged games per day, because people will dodge when they ordinarily wouldn't.
B.) Want to make sure you get mid? Autolock mid even if someone else already picked it. BAM, someone on your team will use their free dodge, guaranteed. Trolling doesn't decrease; it increases dramatically as a result.
C.) Making it easier to dodge tends to put people in a less compromising mood. If you know you're stuck with the four people on your team, you have an incentive to communicate with them and compromise. This is how champ select is supposed to work. If everyone has a Get Out of Game Free card, each player is more likely to be obstinate just because they know there's less chance it will backfire and result in a disadvantageous game that they have to play out.
D.) Offering free dodges encourages the idea that you deserve a smooth-as-silk champ select where everyone gets their role and the team comp is ideal. This will make you feel like you're really justified in getting angry when you have a hiccup but can't dodge without penalty.