I answered the question off my opinion and gave my simple reason based on pure stats, and did not have to go beyond that. It is rare to see a Pokemon with ALL stats above 80. Its specials are at 100. So, Venusaur is underrated I think, especially post Gen.4.
For your questions (if you wanted to post a separate question or 7, you could have) - it can be both offensive and defensive. It is all about how you use the Pokemon with the moves you teach. Your role for Venusaur may be completely different than mine. Simple Pokemon math will determine who beats it and who it can beat.