I don't think that's an issue. Downloading a partial is a problem on private trackers since there are so few users, but on a public tracker, someone downloading a partial is just making the swarm a bit more robust: they are sharing connections details to other users in the swarm and are able to partially seed part of the content.
Hit & run torrent users are the bigger problem; they add nothing to the ecosystem. But, for example, if there's a "complete early roms for all systems nointro unzipped" torrent, and someone only downloads and seeds the SNES section, then the swarm gets the benefit of someone sharing that section of the content.
You could even get a situation where there are no "seeds" but 100% availability, with different people sharing different sections.
I'm not fully looped in to why Anna's Archive did what they did, but their massive 1TB+ torrent zips are pretty useless for most purposes. I'd be happy to download a partial and seed books in, say, a particular genre, but I'm not going to seed a partial of a massive zip file that's useless to me without the full archive.