Well, couldn't you just as easily ask why someone would "care enough" to purchase a collector's edition over a standard edition, period? People purchase collector's editions because they like the extra content and memorabilia.
If he purchased a standard edition of the game at launch and really enjoyed it, then why is it so unusual that he would purchase a collector's edition for the OST, "making" DVD, artbook, and other stuff at a later date? Those incentives still retain their value. Their value isn't limited to the standalone value of the game itself.
Right, my question is: if you want that stuff, why not just buy the collector's/limited edition right off the bat. Or, if for whatever reason you don't (too expensive or whatever) why keep the regular edition after you've bought the collectors?