While the difference appears insignificant, when you clamp down the skewers or nuts, the non-parallel dropout faces will introduce a bending stress in the axle. If you're using a standard freewheel, that's going to make the axle much more susceptible to bending when you hit a bump, pothole, etc.

Once you've bent an axle, have you ever noticed how the subsequent axles seem more prone to bending? That's because the the original bent axle misaligned the dropouts and most mechanics do not realign the dropouts after a bent axle. Big mistake, if you don't want a recurrance.