Well, some fliers would argue that it gives you more control, by having to move the handles less...
Also, in light winds, such long handles enable you to really effect the angle of attack.
IMHO, I've always felt that handles of such length provide more potential over control, and have chosen instead to do a little more "working" of the kite through arms, shoulders and foot work... I won't say it's better, but my quad comp record certainly says something about it.
If you take a poll of the successful quad line competitors, I'd guess that 80%-90% of them use 13" handles.
One thing for certain, extra long handles DO allow you to whip the kite around a lot more... Axels, Flic Flacs, etc... All much easier, and more consistent.