My buddy did his first two tris in some TYR Jammers from REI. I have Pearl Izumi triathlon shorts and love them to death. I did my 1st duathlon in my bicycle onsie and ran just fine in that (plus I had many layers of clothes on becuase it was cold.

It can be preference. For my buddy I think it's just ignorance, or "ignorance is bliss". He hasn't used triathlon shorts and doesn't know any better. He may try on some tri shorts and love them to death.

I will say that triathlon shorts are no where close to good for mountain biking.