A quick grep shows that it's only used in determining which features are supported. However, http://jonnyjd.github.io/libdiscid/discid_8h.html#ad814d490d9037d2a967b835b58649d0c says that discid_read_sparse "Read[s] the disc in the given CD-ROM/DVD-ROM drive using only the specified features."
https://github.com/metabrainz/libdiscid/blob/master/examples/discid.c uses "discid_read_sparse(disc, device, 0)" so DISCID_FEATURE_READ is not used but still the TOC gets read successfully.
So either the documentation of discid_read_sparse is wrong (because you can't disable DISCID_FEATURE_READ) or discid_read_sparse shouldn't read anything if "features" is set to 0.