We use fonts from Data Deja View to symbolize our road route number shields, so these are not standard Windows/ESRI fonts. When we create an MXD using these fonts where the DDV fonts are installed, all is good. When we send this MXD to someone who doesn't have the DDV fonts installed, the font for the route shields defaults to Arial. If you simply tell the person to install the appropriate DDV font, then all is good. So the MXD is storing the original font used somewhere, i.e. the 2nd users MXD doesn't retain the Arial designation for the shields.
But what if, for example, years go by and I have a different computer and I never remembered to reinstall any custom fonts. What if I can't remember what font I used? ArcMap stores this information somewhere in the MXD, so wouldn't it be cool if you could get a listing of fonts that ArcMap uses so future generations can be aware of the fonts needing to be installed? It's like font metadata.
I was told this idea has come up before amongst ESRI staff but never got any traction. So if you're interested in this idea, vote it up!