The purpose is to have everything recognizable on the sequence annotated while not having any redundant annotations.
And yes, the annotations I have in mind are positional and functionally redundant--for example, the same PAS domain annotated by both Pfam and CDD.
For the sake of consistency, you may want to consider always retaining Pfam annotations and dropping others whenever they overlap.
This is what I think I'll go with.
The main reason any of this is even coming up is because Pfam is missing a few crucial annotations that are covered by other databases, but using everything at one go then makes annotations of domains elsewhere on the sequence redundant. E.g., for a sequence that looks like this:
---dom1---dom2---dom3---
Pfam annotates dom1 and dom3. CDD meanwhile annotates dom2 but also dom1 and dom3, making dom1 and dom2 redundantly annotated in the process. Because of the fact that Pfam is useful elsewhere with other sequences in the analysis, I am loathe to drop Pfam, and just want to "paint in" the missing domains from other databases. Hence my question.