Now here we have the problem... for large software systems the user doesn't WANT to keep receiving updates over and over again. Some systems you'll be lucky if they will take two releases a year from you, now matter how much you want to deliver the super-whizzy extra that will make all the difference to them. It costs customers to take software, test it, train the users, upgrade the software, iron out any new bugs...

Most users would prefer to specify requirements way in advance and then bitch 'cos they've changed their mind by time you deliver it. If you want to use XP you need to persuade the customer before the programmer...