Most of the talk about "FRBR-ization" (a terrible mis-nomer, but now common terminology) is about creating clusters of records that represent the same work. In fact, I'm of the opinion that the work level is of interest only to a few (for example, literary critics) -- what most users would like to see is the expression level. The expression is also the level that is needed for the various efforts to associate copyright information with bibliographic data.
In many cases, the work and expression are one and the same because the item has only been issued in one expression. For those, the distinction isn't of consequence.
Where there is more than one expression for the work, those expressions tend to take particular forms, at least for books: new editions, mainly for non-fiction; and translations. In both of these cases, I maintain that the expression level is what users want, not the work. (Non-book experts: does this carry through to other formats?)
My usual example of a translated work is Thomas Mann's Der Zauberberg. According to the cataloging rules, the work's title is Der Zauberberg, while expressions in our libraries may have the title in the language of the translation, e.g. The magic mountain. A FRBR-based work display would be something like:
Der Zauberberg. 1924
This would be the work entry into The magic mountain for users in English-language catalogs, and I assume that many of those users would not recognize the German language title, nor want to go through this level to reach the translated version that they seek.
WorldCat has finessed this by keeping the translations separate -- in other words, WorldCat responds to a search with FRBR expression-level records. And I think this is more user-friendly than the work-level record would be.
The other case, that of editions, also argues for the importance of the FRBR expression-level, but the user needs may be different. In this case, the work level will be recognizable to the user, but the information about which is the latest edition/expression needs to be very clear so that the user does not mistakenly select an item that has been replaced or updated by a later edition. Using the Dewey decimal classification and relative index as our example of a work with many editions, WorldCat shows a single edition on its 'work' page, and I assumed that it was the latest, listed as "Ed. 20," "1989." In fact this isn't the latest edition -- there is a 22nd edition from 2003. Users would only find this by going to what seems to be the expression level where all editions are listed.
This shows how hard it is to create a single grouping for all records that serve the users' needs.
Meanwhile, I have another project that will be attempting to connect copyright information to bibliographic items, including linking to entries in the renewal database. Oddly enough, RDA lists the "copyright notice" element as being at the manifestation level, which seems wrong to me. Copyright is determined on the expression, at least for the two cases I have mentioned so far: each translation receives its own copyright, as does each distinct edition. That these may be republished in a variety of manifestations (hard back, paperback, large print, etc.) does not change their copyright status.
We cannot, however, link copyright information to works. There is no copyright in Der Zauberberg or in the Decimal Classification as a work; copyright will instead be on each expression. So for the purposes of linking to copyright information, it seems that we would ideally have a way to group items by expression. If not, then the only proper link would be on the manifestation, even though that means some repetition. What will make all of this difficult is that we won't often have a date that we can associate with the expression, only with the manifestation, and that isn't necessarily the copyright date. (Except when it is, of course. You librarians reading this know what I mean.)
It still baffles me that we don't include a transcription of the copyright statement on the book or item when we create library bibliographic data, considering how useful that could be. Yet, when I proposed the copyright statement field for the MARC record there was great opposition. Some things I just don't get.