When you create composite versions of legacy diacritical marks it does not populate the legacy marks with anchors to match the “comb” versions.
The legacy marks don’t need anchors.
a) the legacy marks don’t need anchors
b) the anchors are implied by the component. All the component’s anchors are there as well. You can test this by decomposing the component; its anchors will appear as well as its paths.
For finding composites: this is a good case for a smart filter in your filters sidebar. I have a smart filter “composites” set up with the following conditions:
Okay, to be honest, I have “aligned composites”, “non-aligned composites” and “mixed paths/components” set up as smart filters, but I’m sure you can figure out the conditions for those.
I guess “need” in this case is relative for me. 
I switch between Fontlab and glyphs often and Fontlab needs to be configured to use the comb versions for their “auto layer” feature (which is just composites in glyphs). When glyphs files are interpreted by Fontlab, because they don’t have anchors, it breaks the setup of composites by default. (I full well know that it isn’t pertinent to change one’s software to work better with another. I wouldn’t ask that for my case. I figure more context might help explain why I wrote about this.)
The reason it would be helpful for me, and potentially other users to have it, is because I have run into software implementations where the legacy version of diacritical marks is used to create a composite glyph within their software. Which because it lacks anchors causes issues.
Just use the comb versions in the composite. That can be done automatically in Glyphs.
And what are you using FontLab for? (I’m just curious)
That being said Fontlab has a lot of focus on being fully equipped to solve many problems directly.
First, the kerning experience feels more complete in fontlabs. There is so much nuance to what and how you can change things around. This classes panel is a must for quickly reviewing a font.
Second, a lot of functions for Glyphs that Fontlab has is done through third party support instead of first party.
Third, I typically need to quickly evaluate fonts. Though its faint you can see the bearings in the font view for example. You can change them in the edit view by dragging them (which is very helpful for written or cursive fonts). There is also direct implementation for things like tunni lines. Points also can be quickly dragged along the surface to get the shape just right very quickly for organic fonts.
The long story short is that they have a lot of very visual solutions to things and a lot of granularity along with powerful tools that help quickly solve specific problems. If there is a problem I can find it fast and solve it. I can also do a lot to exported files with Fontlabs which makes it compelling to export from.
To be honest Glyphs is much much better for designing from scratch. The tools that are made for it by the community and the focused design approach do make things simpler and quick. I often find myself doing review in fontlabs and making things in Glyphs. It is really hard to say goodbye to the fully featured Fontlabs and the powerful and efficient design of Glyphs and its community. So I just use both. 
Then you should be able to configure that third-party software to use the correct compositions. Legacy marks are not supposed to be combined. They are spacing, they are incomplete (only 13 of the many marks have a non-combining counterpart), and they really exist for historical reasons only.
However, if you do not have those comb marks in your font, Glyphs will default to the spacing non-combining legacy marks. However if you do have the comb marks, of course Glyphs will prefer the correct composition.
But again, the better solution is to use proper compositions everywhere. 
Rainer, Legacy accents are used very often. For instance they appear whenever I want to type a capital letter with an accent. Those letters are written with two keys: first the accent, then the letter. So to type Š, I type ˇ followed by S. And ˇ is displayed. If the font doesn’t have legacy diacritics, you won’t be able to type Š in some applications; in others, the selected font will switch to a fallback font. Legacy diacritics are also displayed on virtual keyboards.
The usage for keyboard input is kind of a legacy thing.
And it is the only reason they are still needed (other than that some of them are part of old encodings).
@filipdesigniq I know, but this way is only true for those thirteen legacy marks. Do it with other marks, and both Mac and Windows apps use space + combining mark (as it should).
Even for those thirteen, I suppose it is a chicken and egg problem. Putting myself in the shoes of Microsoft or Apple, I would not yet dare correct it because there are still too many fonts without combining marks.
For composites like adieresis, eacute and otilde, please use the combining marks. Also include the legacy marks in your font for compatibility. I recommend using the combining marks as components in the legacy marks (which also is the default for Glyphs).
What do you mean by this?
Can you explain that more?
