Thanks for both comments. This is actually quite an old link that has been shared when I linked my Delicious account - so I've just watched it again.
The value of the size of your Twitter followers does depend on what you are using Twitter for. If you are just sharing with friends then it might seem odd to follow people you dont know. I wont go in that but plenty of people enjoy doing just that.
For those looking to be active and succesful in Social Networking then success is really down to the collecting and sharing of information and people. Why would be people do this? Ultimately to be succesful or better themselves in some way. The likelyhood when you follow someone is that a good percentage will follow you back. Therefore the bigger the follower group the more chance you have of making a difference - even if they dont follow you back you still have the opportunity to benefit from their tweets. Jesse makes these three points in the video to sum up nicely the benefits of following 15,000 people effectivly:
Network, Learn and Help People Be active in your community Benefit from this interaction personally - it all comes back to you