Personally I prefer to play him mid lane, and I never liked playing him top lane. I love playing him but I hate the idea of abusing the fact that I'm range over melee, so I don't play him top lane unless I was facing another range top laner like
or any other mage/adc top lane.
I like him mid lane also mainly for the fact that it's much easier to actually 1v1 your laner as most mid laners are squishy champions and you aren't forced to just sit there and slowly kill a tank that builds full MR against you. At least with mid lane you can actually have kill threat and kill them without the need of your jungler. Also as you mentioned, it allows you the better option of roaming, and makes it easier to collapse if a fight breaks out in the river/jungle. Then there is also the fact that playing mid lane Kennen would free up top lane for another good split pusher and you don't need to take that role and can stay with your team and not have to rely solely on a lucky teleport flank.
Though the biggest reason why I feel he isn't played mid which was already mentioned was his weak wave clear which is generally the strength of your mid laner. Match ups can be a hit or miss, but personally I feel his match ups mid lane aren't as bad as people were saying. What people should remember is that Kennen is a flexable champion when it comes to builds. While it is generally ideal to rush
into
and in most cases, you would do that...Kennen isn't punished as hard for being forced to counter build as most mages since he doesn't use mana and is not some one who needs early CDR.
If you are playing Normals, then screw what your team says and just play him mid if you want to. He's viable there and I feel can be really strong when you get the right match up, though I'll admit he is still better top but to me hes more fun mid. Now if this is ranked, then maybe I'd stick with him top lane.