Well, I think you can argue for both options, but I think inline CSS would have caused a lot of problems.
It would have changed the align for all RTL websites to left after an upgrade.
Also in a corporate environments it's important to have the possibility to restrain the admins layout options and since there is currently no way to switch the module align setting off, forcing alignment from the skin is the best option IMO.
I agree having .DNNAlignright with a property of text-align:left seems strange on the other hand a class name like DNNDefaultalign would be very unclear too.
At least it's clear you are overruling the default DNN align.
By the way we did consult some RTL users and this lead to the conclusion that inline styles was "not a good idea"...
I think we should make the "Not Specified" option default by the way.
And default.css, well that's another story.
I agree it's bloated but IMO this was the best solution.