I wouldn't say that the work that went into this isn't worth $20, but it's pretty annoying that QuickLook doesn't support .ai files without a plugin, since the entire OS X display system is PDF-based.
A better solution, going forward, would be to save Illustrator documents in "Illustrator PDF" format, which should display in QuickLook with no plugin.
They did do a decent job on the plugin, but I'm really not looking for lists of colors used in the document and things like that -- I just want QuickLook to display .ai files the same way as any other image file format (other than gradient meshes, which are apparently never going to display correctly in non-Adobe software -- unless those are part of the PDF standard that Adobe just submitted to the ISO).
The clincher is that this plugin does not understand layer visibility, so all the layers I have turned off show up in the preview. This makes it borderline useless.