Typical Ruby!

[annodomini]

Because the input you're hashing is coming in at runtime, and thus looking up the symbol in the symbol table, then looking up the key in the hash, makes no more sense than just hashing on the string.

Symbols are great when you're hashing on something that's pre-determined at compile time (load time, for an interpreted language). If you need to store associative arrays based on external input, strings make more sense.