I think that the magic items will be so different from previous versions that the few examples that we have in LMoP, HotDQ, and the DMG pdf will not be sufficient for some people (myself included). Sure, a DM could do all of the work to create magic items (and quite frankly, a long term D&D game with zero magic items is just not worth playing IMO), but there will be magic item mechanics that are brand new that a DM might not be privy to without the DMG.
I think that playing for an extended period of time without the DMG will be a somewhat subotimal experience due to all of the new mechanics and flavor that the DMG will bring to the table. Just like I think that playing for an extended period of time with the basic rules and without the PHB will be a somewhat subotimal experience. Strictly necessary? No. Necessary for a better all around experience? Yes.
I think that playing for an extended period of time without the DMG will be a somewhat subotimal experience due to all of the new mechanics and flavor that the DMG will bring to the table. Just like I think that playing for an extended period of time with the basic rules and without the PHB will be a somewhat subotimal experience. Strictly necessary? No. Necessary for a better all around experience? Yes.