There are two reasons to send a hero mid for the levels or for the farm, so gankers and carries. A lot of melee carries can't really compete with usual mid heroes though, so you're gonna want to send those with supports unless the player is extraordinarily good.
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Very often, it is a strong ganker or semi-carry that can lane well without support and loves having a level advantage... Pudge, QoP, PotM, Nightstalker, Clockwerk, Tiny, Shadow Fiend, and many more. Rarely, you will also see a support + carry go mid, but this doesn't happen often.
For me, anyone who is level-dependent should go to the middle lane. Bane for example, should get to 6 as quickly as possible. Not only can he kill his middle opponent quickly, but he can easily gank either top or bottom with ease by just using his ultimate.