All it says to me [when a champion is nerfed and is suddenly no longer played by anyone] is that the champion was popular because of some perceived aspect of power, not because people genuinely enjoyed playing them. When a nerf comes into effect and, say, pros no longer pick the nerfed champion in tournaments, 99% of solo queue (which is very different and where picks are less important) reacts and follows suit.
How well a less frequently picked champion can be used depends on their objective potential (which is, or should be, very similar across the board), and how well you use them (strategically and mechanically). One advantage of choosing Rammus in your case, for example, might be that half of your opponents won't know or remember exactly what his abilities do, while 80% may have never even played against him before. The advantage of an unfamiliar champion choice should not be taken lightly.