It will because of the Blu ray read speed which is the same reason there have been PS3 exclusive games with mandatory installs.
MGS4 didn't require an install because the 360 uses DVDs.....
Yeah it will be for the BR speeds, but R* had to do the game on the PS3 as well as the 360. with the game being so massive they needed a solution for the 8.5gb space limited on the the 360's dvd's so instead of just putting the game on 2 discs and not require any sought of install on the 360, they probably thought we might as well have the install mandatory for both platforms so we can use the advantages of having 8gb stored on the HD, the game world being so massive and complex the game may require the install just to make it work..........