The idea that was behind the feats was since the double scimitar is the signiture weapon of an incredibly combat-focused culture, it would make sense for it to have some unique feats for the weapon. The finesse feat was mainly because the double scimitar seems like a weapon that you have to have fine control over to use, since you have to spin it around to take a second shot with a blade. You can't cut with the back of the blade, so you have to be able to control it presicely, especially since you have a second blade to keep control of on the other end.