Someone not wearing a shirt (not sure where you live but that isn't uncommon in the summers where I live), or tank tops which will make many gi techniques useless (ripping and just not having much material to work with), tight t shirts (or even baggy shirts often will just tear easily), etc. Like I said there are uses to gi training, and it's great to have gi options in case you are in a fight with someone wearing a jacket or something (common in the cold, wet winters where I live) but I definitely think no-gi is more important and should be more frequently trained because I believe in being equally effective in the situations where you have fewer options (being good with your wrestling grips, clinching, etc. when you don't have extra hand holds from thick clothes). no-gi grappling skill translates better to gi/clothed situations than gi skills translate to situations where you don't have gi techniques available.
Member of DaJoGen MMA school under Dave Hagen and Team Chaos fight team under Denver Mangiyatan and Chris Toquero, ran out of Zanshin Martial Arts in Salem Oregon: http://www.zanshinarts.org/Home.aspx