I think you should be able to double-click them to load the file as a normal AI document, then do a 'save as' and choose some lower version. I did a quick test with some of the brushes that came with AI and it loaded into the brush palette just fine.
No comment on the actual brushes as I'm still using CS2.
Me, I have a grand total of four custom AI brushes that I put in my default documents, as I got tired of creating them over and over. One is my signature chop as a scatter brush; I just drag it from the brush palette to an empty place in the document to put my signature there.
The others are art brushes. It is easier for me to describe them than to bother uploading them. Seriously.
(If you don't use Illustrator this will be completely useless to you; skip over it.)
- set your fill color to 100% black
- grab the ellipse tool, click with it. tell it you want a 10x1pt ellipse.
- drag that into the brush palette, make it an art brush. set the color mode to 'tints'.
- select the leftmost point of the ellipse, delete it.
- grab the remaining half-ellipse and drag it into the brush palette. another art brush, same color mode.
- grab the rectangle tool, click with it. make a 10x1 pt rectangle.
- zoom in tight. select just the vertical line on the right side - no points, just the line. hit the delete key.
- select the two rightmost points, do apple-alt-shift-j. this is an undocumented shortcut that does the same as hitting object->path->average followed by object->path->join. it's probably ctrl-alt-shift-j on windows but I never use that. Anyway, you'll end up with a triangle.
- drag the triangle into the brush palette, art brush, tints.
Use for stray hairs, eyelashes, outlines, linear highlights, etc, etc - anywhere you need a mostly linear element but don't want it to be the boring ones of a normal line.
I make them in this technical way so that the weight behaves as I expect it to; if you draw really big art, you constantly have to type fractional values into the stroke weight box.