...maybe? There's no guarantee that the back of the dropouts are aligned perfectly enough that the wheel will end up straight if you do that. And the ideal spot for the rear wheel is somewhere in the middle of the dropouts anyway -- putting it back further increases the chain gap, and having it too far forward reduces the wrap angle on the freewheel/cassette cogs. Putting in a dropout spacer (which is probably cheaper than drilling/tapping the dropouts for adjusting screws and supplying said screws) is a convenient way to make that middle position the default, while still allowing the user to align the wheel.