FPTP = What flavour Ice Cream do you want? If its not available you'll get what your given.
AV = What flavour Ice Cream do you want? If its not available what flavour would you like instead? And if thats not available what flavour would you like instead?
For voters there is no downside of still having a choice even if your first/second etc choice isn't available.
For political parties the advantage of FPTP is that you only get once chance so you have to 'take it or leave it' they don't need to put themselves out to attract your vote
25th August Addition
Checkout my short (1min 23sec) animation demonstrating the above on YouTube http://www.youtube.com/watch?v=WZzfZ9hCxq4