Originally Posted by
Phil_gretz
...Someone please explain to me why I would want to/need to use my big-big combination in favor of dropping down to the middle range of my lower chainring. Please?
To avoid dropping the chain on a climb, especially undulating rollers, during a race. Happens more often than we realize, unless the race announcer sees and mentions it, or we're eagle-eyed enough to spot it ourselves. In a group ride or race you can hear someone's chain drop.
Chain drops seem to be more common with bigger gaps in some chainring combos -- 53/39, 50/34 -- and index shifters, than with old school 52/42 and friction shifters. But front derailleur shifts have always been trickier than RD.