In the last few years internet has become a paramount center of any desired install of any operative system, without a good internet experience computers are seen as barely usable by most users.
Many new users come to linux/ubuntu/xubuntu because they feel their old computers would improve their experience, either by providing a safer enviroment promoted by a linux "virus free" web experience, or simply to have more up to date software. However the Web Browsers that today's systems provide have a heavy need of memory to work properly. In a simple test, firefox 22 consumed 342MiB while having just 2 tabs of the ubuntu wiki open.
Taking into consideration that websites like facebook or youtube would require much larger amounts of ram to stay open and not freeze.
Given this issue the proposition is to rise the amount of RAM indicated in http://xubuntu.org/getxubuntu/ to 1GiB as the minimum recommended. While nothing that lower amounts of ram "will" work, they wont provide the most pleasant of experiences to users, and that if they do use lower amounts of ram they will experience problems that would seem like bugs (like freezing of the web browser or other related errors), but wont actually be so.