There really is no purely offensive or devensive martial arts out there. What it boils down to is the mindset you have going into it. Take Muay Thai for example. The fighting style is very aggresive, and it can inflict massive damage in a very short period of time if applied correctly. Knowing what I know about it I can say with little doubt, that in a street fight I would probably be able to wipe the floor with someone in short order. Knowing that, I DO NOT go out looking for trouble. If I did I would be using my training offensively and that goes against what I have been taught. Mearly being prepared and knowing that I can handle most any situation that might come my way is devensive in nature and that is why I train.
The mindset of the student and teacher is more important than the type of martial art that is practiced. I am a firm beliver in some form of mixed martial arts though. The more knowledge you have the more prepared you will be when a situation dictates that it is time to use what you have learned. You never know when some drunk a hole at a bar or some punk kid will take you for an easy mark and start some crap. Eliminate the initial threat quicly and decisively and any buddies they have will quickly think twice before they take a beat down.
Just my two cents.