Assuming Xbox 360 owner has a PC with a display bought within the last few years, they can use that with the appropriate connectors. VGA if you're working with old equipment, HDMI to DVI cable if not.
I don't really get these arguments about "having" to "upgrade" TVs to see HD resolutions considering you technically also have to "upgrade" your PC monitor to see the same or better resolutions, assuming you started with a 640x480 VGA CRT monitor from the early 90s. Most aren't. They already have "HD" compatible resolution PC displays whether they know it or not.
Do you have a PC display over 640x480? 1680x1050 displays run what, about $150 if you buy the cheapest one? Even the cheapest new PC display you buy today would have no less than 1280x720 resolution if you spend at least $100 on it.
Point is, if you have one (HDTV or non 90s era PC display) you can use it with both your PC and your game console, assuming you know how to buy and plug in the appropriate cable. It's tough, I know. Practically rocket science, but there are a lot of gamers currently using their HDTVs as PC monitors, and PC monitors as console displays.
Technically, you don't have to buy/own both if you're on a budget.
I have a PC and an HD console attached to both PC displays and HDTVs. For the record, I am not a rocket scientist.
But what about those who have no PC (and no PC display) and only have an SD TV? Life is tough. I think there are about 6 billion people who don't own an Xbox 360 either.