As stated it really depends on your list. With all of the new S6-S8 non-melta spam out there, glances and then stunned results are quite common (at least for me). On something like a landraider that MUST keep moving to get its payload on target extra armor is critical. With such a high armor glances are common, and so are stuns. Same goes for dreads who want to get into CC on the next turn. Vehicles that hang back in more of a shooting support role certainly won't need it.
It also depends on the army. I know for my BA, I like to put extra armor on a few since my vehicles are also fast. That means last turn objective dashes can mean getting that win or tie. A vindi is a good example here for BA. Since it can move 12" and still fire, and with its relatively short range, being able to move after a stun can keep it alive to shoot another round and/or avoid CC, tank shock, or contest an objective in that crucial moment. I will often use vindis and/or baals as a mobile wall for my jump infantry so if they can keep moving up for my jump packers, extra armor becomes an invaluable piece of kit. Another example would be BA razor back spam. When you have so many small units in razorbacks, you want to make sure they can stay in them and move to those objectives late game. Foot slogging 5 guys on foot more than 12" on turn 4-5 won't always get you on those objectives.