Purpose of partially filled in rear drop out I have an old MTB frame that has the drive side rear dropout partially filled in as if it was part of the original forging of the dropout.  And, I have seen this on a few other frames -- even different manufacturers.

Clearly it was done to limit how far back the axle can go in the dropout but the question(s) is(are):

why does the frame maker want to control this detail?

why is it only on the drive side?.

=== edit === 

To clarify...

This is not the bike in question but it is an example of what I mean. My apologies to the site where I found this -- it was not copyrighted.
What you more often see is that the derailer hanger is a separate piece that bolts into what is the filled-in space in your picture.  

But this is for frames that are designed for the axle to fit only that far back.  Generally these are less than top quality frames where having both dropouts match exactly would make it obvious that the frame wasn't quite true (since the wheel would not center with the axle all the way back).

So with one side filled in you are forced to tighten that side, then slide the other side forward or back until the wheel is centered.  They make it look like a "feature" that facilitates alignment.