I wouldn't be surprised if you could. And with DC, you can willingly not share anything, and I'm pretty sure there are BitTorrent clients out there that don't upload, too.
However, in my eyes, as soon as you do this, you are no longer using peer-to-peer services. The idea behind them, to me, has always been a "shared" responsibility for the workload. If you just download from a network, then I would count you as a regular downloader, or user, of a server (or network of servers) and no longer using peer-to-peer services (as you are no longer acting as an equal).
I mean, sure, you can download from a service that "internally" runs as peer-to-peer. And by "internally" here I mean everyone who contributes to the service is contributing some of the workload, but that doesn't mean you are also acting as a peer.
For example, NTP runs as a peer-to-peer service, somewhat, in that many servers around the globe communicate to keep an accurate time. But if I sync my clock to some NTP server once a day, that doesn't mean I'm a peer. That just means I'm a user. Similarly, DNS runs, between servers, as a peer-to-peer service. But just because your browser does some address lookups doesn't make you a peer.
But that's just my take on it, if you're just applying "P2P" as a label, to all users of a given protocol or program that is aimed at P2P services, then the label by your own definition applies.