Hill's rationale? Make it even easier for them vote, and it could add an extra percent or two.
The unions would love it anyway, since it gets them another day off at their employers' expense.
You guys are missing the point. If you give people the day off, then it is easier for you to get your supporters (unions, etc) to politic for you (phone banks, drive people to the polls, etc), and they don't have to take a vacation day.
See, you only need a couple of hours to vote once, but the possibilities are limitless with the whole day off.