Here’s a fun little brush I just whipped up in Illustrator.

1. Draw a 1-pt line.

2. Go to the appearance palette, click on the ‘fx’ dropdown. Path->outline stroke.

3. fx->distort & transform-> roughen.

These are the settings I used, go with whatever makes you happy.
4. Drag the ‘outline stroke’ entry in the appearance palette ONTO the ‘stroke’ entry.

5. Drag the ‘roughen’ entry to just above the ‘outline stroke’ entry.
6. Drag ‘outline stroke’ ABOVE ‘roughen’.

I tried other ways to make this stack of effects happen, but this is the only one that would reliably and repeatedly work for me – I’m pretty sure this is some kind of bug.
7. Now you have a cool brush with messy edges! This is a 6-point dark purple stroke, with two 1-point strokes slashed in over it in a lighter color.

I may never draw a squiggly highlight by hand again. I’m making a graphic style of this and sticking it in all my startup documents.

BONUS ROUND: PRESSURE SENSITIVITY!

Make a simple calligraphic brush. Now you can use the brush tool (B), vary the width of your squiggly lines with pen pressure, and choose different “sizes” of brushes with the stroke width panel.

You could probably do a bunch of other interesting things with this method of outlining a pressure-sensitive stroke, then applying live effects to it.

Stylize->Scribble looks pretty cool, for instance.
Or how about stylize->distort and transform->pucker and bloat?

Fool around, stack some effects up, experiment. If you were really dedicated you might be able to produce a really fun set of janked-up styles.