If ems essentially give you a result that is a percentage of the base font, what advantage is there to using it, rather than % for font sizing? I have looked around, and haven't really seen anything that implies that the two behave differently from each other. Is there a cascading issue?
EM and PERCENT are both very similar, the only difference between the two can be observed when changing text size on the client browser.
In theory, the em and rem units are the new and upcoming standard for font sizes on the web, but in practice, the percent unit seems to provide a more consistent and accessible display for users.
Original article by Kyle Schaeffer: http://kyleschaeffer.com/development/css-font-size-em-vs-px-vs-pt-vs/
In terms of behaviour, there is no difference. The difference cited above is simply a buggy browser implementation, in my opinion.
I just did a quick test in current Chrome and Firefox, and there is no difference in sizing of child elements : smaller; : larger when the parent is either 1em or 100%. I would very be surprised if this bug was present in any modern browsers; browsers have come a long way in seven years.
The only difference, in practical terms, is the extra character required to use %. Why use 4 when you can use 3?
Beyond that I believe the only difference is the historical precedent. There are hundreds of years of tradition behind setting type size in ems.