POI would encourage people to spend NEM because just hoarding will not link your account to others. POI considers not only the amount of NEM (as in POS), but also considers the structure of the transaction graph. Thus nodes that are structurally important to the health of the network get a higher reward.

Thus POI is kind of in-between POS and PON. If you have only POS, no one will spend. If you have only PON or only consider links between nodes, then people could game the system. By requiring that people have NEM (have a stake in the system) and use NEM (send NEM to other accounts and receive NEM from other accounts), POI promotes using the currency. Now, it might sound like someone could just send a bunch of NEM around in a loop and make their account big, but the coin-days limit prevents that, as well as the fact that only sending NEM in a loop will not make your account(s) more important, because importance will be conserved within a loop. You'll need to actually transact with others in order to make your account important to the NEM economy.