This is one area where the community stance is fairly unanimous. The language tag is the most important. In fact, all questions on Code Review have the language tag.
In fairness, the thought behind the perpetrator is likely that it is a more specific and therefore implies the derived tag. Perhaps they desired an additional tag but five tags is more than sufficient and once you consider the actual way tags are used the mistake becomes obvious.
Say there are people who are interested in Ruby in general, and follow the tag. These would be helpful people are then no longer be able to color-distinguish or filter and find the post if we allowed such a change.