Okay I see the dilemma. I've mapped some hockey arenas before, many of which have multiple rinks and so have a similar issue about names for individual rinks (Rink A, Rink B) vs the arena as a whole. As far as I know there is no good solution yet.

For single non-notable baseball fields with no name I would think it's best to leave the name off entirely, as adding a generic name wouldn't reflect reality and wouldn't help anyone find the field. And for fields notable enough for wikidata pages, noname=yes seems better than adding a generic name. That's just my opinion though, I'm not by any means an expert in this.

Re: duplication
There are now overlapping ways for leisure=sports_centre https://www.openstreetmap.org/way/36907037 and leisure=pitch https://www.openstreetmap.org/way/36907037. I think there should only be a pitch. The wiki for sports_centre says it's not meant for single fields.