If nothing else, validating opens up possibilities. If you're an academic rockstar, it could help you do VGEP (voluntary graduate education program: starting 2nd semester 1/C year, you start work on a Master's degree at a nearby school). If you want to minor in a language, validating a couple of semesters makes it a lot easier (or, for group III types, can help eliminate the language requirement).
Also, though the minimum number of credits per semester is 15, no major actually has all 15 credit semesters. Getting ahead plebe year would mean that later on the 19/20+ credit semesters suddenly become much more manageable 15-17 credit semesters.
Since I need to take my shoes off to count past ten, I only validated a semester of English. But doing that allowed me to get (slightly) ahead with some other required courses.
The cons are that you might get ahead of yourself. I know a lot of people who validated out of Calc I and sometimes Calc II as well and ended up getting swamped in Calc III or Diff Eqs.