It's a tradeoff. On the other hand, you can more easily implement backup systems that copy directory trees, using hardlinks for files that haven't changed. For example.
I don't know the details of how specific filesystems are implemented, but it seems that if it's reasonable to achieve the locality you want, then it can be done for the first name a file has. Subsequent names wouldn't have good locality, but second and third links to a file are much less common. If you want other links to the file to have good locality, then simply make a copy, doubling your space requirements.
Hard links are useful, and you don't necessarily need to sacrifice locality in the common case, and in the uncommon case you can still choose between good locality, or good use of space and the sometimes useful semantics of two names by which to read or write a file.
I don't know the details of how specific filesystems are implemented, but it seems that if it's reasonable to achieve the locality you want, then it can be done for the first name a file has. Subsequent names wouldn't have good locality, but second and third links to a file are much less common. If you want other links to the file to have good locality, then simply make a copy, doubling your space requirements.
Hard links are useful, and you don't necessarily need to sacrifice locality in the common case, and in the uncommon case you can still choose between good locality, or good use of space and the sometimes useful semantics of two names by which to read or write a file.