After the NGS migration there are plenty of releases where the recordings weren't merged in the migration (because they weren't linked properly or because of track time mismatch). Example:

http://musicbrainz.org/release/dd49e02b-0426-446f-b240-e3977a18d60f
http://musicbrainz.org/release/1809eeb1-11b9-4a5e-a021-7ede9b1ab448

For these cases, it would be extremely time-saving and less error-prone if it were possible to merge only the recordings without merging the releases themselves.

One way of doing this would be to expand the merge releases UI to add a third merge strategy: "Merge recordings only". Of course it might be a bit unintuitive to merge recordings by trying to merge the releases, but I don't have a better suggestions at this time.