Which you're not allowed to do on the only platform where it matters, because MS once sold a model of the 360 without a HDD.
They did it with forza,and refering to "once" as in the past,makes no difference any-more. 95% if not higher of xbox users have a hard-drive. A game could be done on multiple disc's while still requiring one to use,just like a single disc game. Microsoft has also shown,blu-ray is still not needed on this/these gen of consoles either. Some games on the xbox have also recommended it's better to install to hard-drive.The space is there,use it.Playing from hard-drive is much better anyway.
This also shows the importance of current techniques,with older tech. It shows progression can be made without buying new tech all the time,if that makes sense. An example of this is how they squeeze what they can out of the two consoles,older tech,new tools/technique.Because the tech gap has been so long from when consoles were first around ( 360,PS3 ),when we do get the gen of consoles,the gaps been that long ,that it will warrent the change,and be all the more worth while,even though that tech is around now. This example can work in many ways not just gaming etc.