as I see it:
optional comments:
con: you don't know why someone voted...
con: a bad person has to do less trouble to make a 'spam'-vote (just click the button, et voila...)
not-optional comments:
con: less votes will be given (face it, people are lazy

)
con: more votes without meaningfull things (like: "good", "I like") <= that actually a very small negative point... I can live with that...