Rider preference plays a big part in whether you need bib liners for mountain biking. For those that like to go on long rides then bib liners are a worthwhile investment in order to keep you comfortable after many hours in the saddle. While comfort won't play as much of a role on shorter rides, it's still important. For shorter local rides or a few enduro laps where you don’t need to carry much, having the extra cargo capacity means you might not need to burden yourself with a backpack