I do happen to have a light in my coop, but it's not for the chickens! I don't supply them with supplemental heat or light. But sometimes during the cold, short days of winter I might find myself having to go out and finish up last minute chores I didn't get done during the hour and half of daylight we have. (Just kidding on the length of daylight hours, but sometimes it sure feels that way!) Anyway, the light is for me. I try really hard not to use it - poor chickens would just get settled on the roost and I'd come along and plunge them into bright light. It's just nice to know it's there if I need it.
Hubby is an electrician, and we knew that eventually I'd want to brood chicks out there rather than in the house. So as long as he was already putting in an outlet for that, he just added the light. The right solar light should be okay, but as others have said it depends on what you want to use it for and what size/kind you'd want to use. Since I've only turned on my light switch one time in the whole year it's been up there I don't know that it's "necessary", but it could be a nice touch.