Instanced content gets "gimped" when those who do it a lot (solid Raiders, etc.) need mats to craft things their group will need, flasks, food buffs, el al). If they are using ground travel when free-form flight is available they will suffer a slower acquisition of those various mats and thus slower crafting of those things, and thus slower progression through that instanced content... versus those who use flight. So, any 'competition' between guilds about server firsts and such (yes, I know those are being removed but doesn't change the competative mindset much) automatically puts the advantage to the side that flies -- thus flight becomes "mandatory" and not using it is a needless "gimp".
There are others if you would just think about it instead of knee-jerk denying the possibility.