Why spend $100 on a nice pair of well designed *seamless* cycling shorts if you are going to add underwear elastic..... They are designed to go without.

You can ride without cycling shorts and you can ride in underwear, and you might even get away with it comfortably for a time, but the longer and harder you ride the more you'll find that it can be a bad idea. When I was a kid I did week long tours in jeans and regular shorts and survived, but when the technology to be more comfortable is there why not use it and use it the way it was meant to be used. Trust me - if you ever get rubbed by a seam/elastic or get a saddle sore you'll toss the undies away and never go back.

And for those who wear underwear to not have to wash their shorts as often - do you honestly believe a pair of undies keeps the sweat and beasties off your chamois............