Comments are not necessarily free. They can be costly at maintenance time. There are many occasions when the code is changed and the comments are not...especially when the changes are not done by the original developer. An enum forces the code to use a value from a limited set and still allows for good readability.

An enum does cost more at compile time but is translated to an integer value at runtime. I would think that if an enum was used in a stored procedure that is compiled once, the performance penalty of translating a character string to an integer value at compile time would be negligable.

An enumeration also acts as another integrity test through type checking. This, IMHO, is more valuable than the small performance gains from using any type of integer.