Unlocked camera lets you screen check what happening elsewhere on the map (you can temporarily unlock camera to get this even if you play normally locked). What I find unlocked cam valuable for, that locked can't give you, is aiming skillshots and not misclicking. Having everything else move because your champion is moving (which is almost always the case, as sitting still=invitation to death by skillshots), makes landing skillshots and not misclicking harder to do. No reason to add difficulty like that. Keeping the unlocked camera in the right place to see all the action can also offer an advantage over locked cam.
Overall, I believe unlocked cam is better, especially if you have experience playing RTS games as the camera's in those games are generally the style of top-down view camera controls. I played with locked camera for only my first few games, after that I switch (at the recommendation of my friends that played with me) and after taking a few games to get used to it, I came to vastly prefer it. (I can't stand playing locked cam now, won't do it; would play like shit too trying to do so).