Energy ain't free, the additional lights fuck sleep schedule, blackouts may happen, the computer produces heat which wears its own pieces, chances are it will be kept online meaning greater risk of being hacked, computer on means more read-write operations which wear the memory down as Nutin said, and so on.
At most, maybe it'd be justifiable if it's downloading/running something which can't be stopped. Or another possibility though not a justification, the person isn't responsible towards his/her machine. Otherwise, I struggle to think of reasons not to turn it off.