Thanks for your report and for contributing to WordPress core!
There's actually another if/else-block coming after the the else that you mention (six lines of code, down to the line <?php endif; ?>). Thus, the if and else actions are in fact different here.
Now, there is a redundancy with the <description><![CDATA[<?php the_excerpt_rss(); ?>]]></description> line, which technically could be moved out of this if/else check. I don't really think that this brings advantages here though. It might just make the code harder to comprehend.
Of course, if you want, create a patch for this and I'm sure a core committer will take a second look! :-)