The 'normal' sliding rear takedown pin is not useless. With some triggers you will not be able to put the upper on the lower once the trigger is installed. The Geissele High Speed National Match trigger for example needs just a bit more room to slide the upper onto the lower.
If you are a going for a ultra light build, you can also save a few grams by using a Titanium rear pin instead of the supplied steel pin. A normal pin also looks better in my opinion.