Having both IPv4 and IPv6 is like having hot and cold water pipes running through the same single tap. You can't get more than the tap, but somethimes hot water gives better pressure and sometimes cold. You have chance to choose and if hot water(lets say it is coming seperaty from street) stops flowing, you can still get cold water..
If you combine IPv4 and IPv6 p2p seeders, you can maximise download speed, especially when some countries have developed IPv6 only networks. Some providers filter p2p on IPv4, but not IPv6, like here -
http://www.ipv6.com/articles/blog/Comcast-Verizon-Filtering-P2P-Traffic.htm
Diversification, imagine internet gets flooded on IPv4, you still maintain some access via IPv6.
Yes it might be, that if you are very standard user not using any innovative applications, you might not need to upgrade or to wonder another three years, but otherwise I would rather keep up to date, especially if you are investing in new hardware, using p2p & etc.