The GOP wants voter ID laws because they know they depress turnout among primarily Democratic populations. Democrats oppose them because they know it too.
It is a political calculation for which both sides have constructed principled defenses. But it boils down to politics.
Voter ID laws would depress turnout in - for example - Philadelphia. And since Democrats need a big turnout in Philly to win PA, the positions of both sides are predictable.
Don't take my word for it; here is PA House Republican Leader Mike Turzai on June 23, 2012: