mega gengar is an interesting case. for a lot of pokemon you will try to mega evolve relatively early to get the stat boosts, ability boost, etc (char-y, mega venu, mega scizor). for some you wait a little longer throughout the battle to abuse the not-mega evolved states ability until you need it (garchomp's speed, manectric's lightningrod, mawile's intimidate).
mega gengar falls into a gray category where you may or may not mega evolve depending if you need levitate (for your opp's or even your own eq's) or if you need trapping. i would agree that as a standard, being able to trap is nice, but i always consider this one question before mega evolving: "do i want my mega gengar to be
forced to protect on potential eqs?"
if there isn't really an eq problem, then ofc i will mega (like no ground pokemon at all)
if there are multiple eq'ers ill probably wait it out and let the rest of my team wittle down the eq'ers before i send in megagar
if there are 1 or 2 eq'ers, then ill still probably mega evo because im edgy and usually i can pair mega gengar with something that can smack ground types. you gotta remember, you are the one controlling the switching. more often than not, you will be able to force them into a bad position just by having megagar protect while you swap in some sort of check/counter for the opposing pokemon without fear of them switching themselves because of shadow tag. thank you fuckin based shadow tag
one note: potential things that can swap even with shadow tag. scarf u turners/volt switchers (im lookin at you lando-t), ghost types. you won't always be able to trap these buddies
second note: what is a ghost type that can smack mega gar? yeah that's right. golurk. it can swap in and out if it dosen't wanna fight. and if it does wanna fight, then it can stay in and smack the shit out of gar with eq. and you get the added bonus of being a hard terrakion counter! golurk fuckin best pokemon 2014.
oh. mega gar also outspeeds some scarf lando-t variants. that's p cool :^). it's kinda hard to scout though unless you already have a sub up to see who is actually faster. so that's why some people like sub i guess
personalmente i like sludge bomb because base 130 outspeeds skymin, and how often can you pop that nigga off :]
most importantly though, i nick my gars "boo kale"!!!