Using a sequence instead of a map is fine if you know it will never be more than some small number of entries, but so often we are wrong about how many entries will be in a table that the N-squared use of a sequence as a map kills performance unexpectedly.
You don't replace every map in your code with a distributed Key-Value store, in case it might need to store terabytes in the future. Most of the time you have a pretty good idea whether a map has to store a hundred, or a hundred thousand elements.
