I think the best approach to creating naturally looking grungy hand-drawn lettering is by using a combination of Photoshop and Illustrator to do the job.
First I'd work on a black and white pixel-based version of the lettering by either pasting it from Illustrator into Photoshop or by creating the lettering from scratch in Photoshop using either the Brush or Pencil tool. I'd give the letter edges a Gaussian Blur. Then I would apply filters like Spatter to roughen the blur and then use Stamp to make edges harder yet smooth some of the rough detail.
The goal is to create an image that can be converted into vector form and be usable. You don't want to end up with paths that have many thousands of anchor points. When the lettering image looks good enough it can be turned into a selection by opening the Channels panel. That selection can be turned into a vector-based path by opening the Paths panel and choosing "Make Work Path." Sometimes this approach can yield better results than the Live Trace and Live Paint functions in Illustrator. But some experimentation may be required. The resulting path can be exported in Illustrator format.
In the end you should end up with grungy lettering that looks natural. A grungy vector brush in Illustrator may end up repeating the same bits of messy detail in different places. The eye can pick up on the repetitive elements very quickly.
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