This is related to MATSIM-827, but it is not the same, and also the discussion underneath that item veers into some different territory.
The issue here is that one can have, in config.xml,
Turns out that they have different consequences: In the first case, mode params for mode "abc" are added to some pre-existing defaults. In the 2nd case, defaults are first cleared.
(This was probably my doing, not understanding that there were two different dialects.)
Took me several hours to understand that, and in the end I decided to leave it like it is since I wasn't sure if I could decide on a good direction. One option would be to remove the defaults. I like defaults, since they somewhat standardize simulations between users; otherwise in particular new users just throw in numbers until it "works".
But I am not sure if we want to leave it like that.
The consistent way to deal with defaults is possibly the way we do it in PlansCalcRouteConfigGroup: Clear all defaults with the first addition of a user-defined parameter. (The argument for this is tricky, but without this one fails the write-read-test, since if one has something that is a true subset of the defaults, then writing it out and reading it in will have all defaults installed.) Clearing all defaults might be an option here has well.