For me, my disuse of the 3rd strap is on all fours with my overall approach to Highland Dress: simplicity.

3rd strap: serves no purpose; makes kilt hang poorly; makes kilt take longer to put on.

Belt loops: serve no purpose; if used eventually makes kilt look bad in back.

Dirk belt: I don't wear a dirk, so it serves no purpose; another useless thing which takes time to put on.

Kilt pin: serves no purpose; makes holes in my nice kilts; potentially tears a big hole in my kilts if I snag it on something while marching through some crowded room while piping.

As a gigging piper, oftentimes I have to get dressed in a hurry (the 'hurry up and wait' thing) and oftentimes I have to get dressed in cramped places, so the less stuff I have to put on and the quicker it is to put on, the better.