It seems it would make the structure more complex and heavier. I would guess it also would cost more to build a pylon wing glider. Shoulder wings with smooth integral wing root fillets (ASW-15 for example) should also be more efficient than simply mating the wing end on to the side of the fuselage but Schleicher seemed to give this up as not worth the effort when they moved to the ASW-19 (mid mounted wing with a more vestigial fillet).

One of the interesting things about glider design is that, even for open class, it's not really a free for all. The design has to comply with national airworthiness requirements and have acceptable flying characteristics for the average pilot. Compare this to an Unlimited Reno racer for example. I remember what Gerhard Waibel said about the ASW-12 in hindsight. It was something about learning how all new, hot gliders will become older gliders flown by less than top rank pilots eventually and you have to take that into consideration even when you're trying to make a world championship contender..