Why use strings when doing rules for Form Validation instead of flags

[eluser]ocdcoder[/eluser]
Just out of curiosity, I was playing with the form validation library today and realized that the rules (the parameter separated by "|"s) are doing as a string and not as flags. I could imagine there would be some performance improvement if this was switched to flags, or at least passing in an array instead of a string.
Has anyone else thought of this too?

[Edit]
So after looking at the library more, I see the reasons as to why it shouldn't be flags, although I still think passing an array will be faster. Any reasons why not?