You can get to the images from the film number on FS: click the camera icon in the catalog entry. Of course, you'll also need the image number, and I find it helpful to include the "of NNN" part, as a verification that you've got the correct film.
But you're right that now that they've stopped loaning microfilm, the film number is not particularly useful. The _digital_ film number (DGS) is slightly more useful, because you can bypass the catalog and just plug it into the film-viewer URL, but it's still specific to FS only.
As far as I know, microfilm numbering is always specific to the holding institution, regardless of the original source or filming entity. For example, when FS filmed the church registers in Hungary, part of the deal was that the National Archives of Hungary got a copy. This means that the NA's microfilm holdings are exactly the same as FS's, but the archival reference numbers and such are completely different.
Another reason to include the full "image N of NNN" reference is that films that are included in a waypointed collection on FS can have two different "identities": (digital) film number or waypoints. Because waypoints can break films into smaller chunks, the image number may not be the same.
When I had a month's subscription to the Lutheran records in Hungary, I at first tried to keep track of page numbers on the images, because I knew that their filming (digitization) was in totally different chunks than FS's holdings, but I gave up in favor of place, type, year, and entry number ("Szarvas, marriages, 1881, entry 3"), because page numbering was feast or famine: two or three different page numbers in the corners of some images, no page numbers anywhere on other images.
What it boils down to is giving sufficient information for someone else to find the same thing you were looking at. Something like the NARA film number can help with that, because it'll be the same whether you're using FS or Ancestry, but it is neither truly necessary nor sufficient.