I used to roll my eyes at double bagging too, figured it was overkill and wasted money on supplies. Then I bought a run of X-Men 129 to 137 off a guy and every single one had spine ticks and corner dings from just sliding in and out of a single bag over the years. Now everything I own that's worth more than cover price goes in a bag with a board, and those books go in a second bag without one, and I sleep a lot better at night. That guy at Heroes Con was asking a fair question, but he probably never pulled out a key book and watched the color flake off the spine.
I learned the hard way too, after pulling out my copy of Giant Size X-Men 1 and seeing a color chip flake off right at the spine corner. Now everything worth more than a few bucks gets a bag and board, then a second bag over that, and it has saved me a ton of grief. @young.ryan you are spot on about the wear from sliding books in and out, that stuff adds up way faster than people think.