But if it truly has no use case then you don’t have to worry about breaking something others depend on because no one would depend on it for they have no use for it. If they are using it then there must be use cases.
If you add something people will use it. Even it was poorly thought out because one was speculating on the need instead of actually having one. Identifying an actual honest-to-god use-case (preferably three) is the single best way to validate a design. Whether it’s my use-case or someone else’s doesn’t matter at that point.
That’s why I like it when people point out missing things… then I have a real use-case to work from.
So, for instance, every modern UI implementation I’ve used such as JavaFX, Android, GTK+, Unity, Nifty and WPF include a grid layout, such as your SpringGridLayout, which is excellent by the way, as-well as horizontal and vertical layouts such as HBoxLayout and VBoxLayout so you might say Lemur is missing horizontal and vertical layouts.
Edit: Scratch that, actually neither Nifty or Unity include a grid layout, just horizontal and vertical layouts.
For every use case I originally had, BoxLayout served the purpose… but then I just use SpringGridLayout for everything now because it will do box layouts. I think the only thing it misses is that it won’t do your special (new) dynamic double-dispatch layout.
Well it’s a matter of personal preference I think. I prefer working with the horizontal and vertical layouts myself. My guess is that maybe someone more familiar with Swing would be quite at home with SpringGridLayout and BorderLayout while those who got their feet wet with JavaFX would feel more comfortable with HBox and VBox. I never used Swing myself, cut my teeth on JavaFX 
SpringGridLayout works just fine with my dynamic double-dispatch layout except that you can’t center/bottom/right align with it since DynamicInsets broke, but eventually I’d like to modify SpringGridLayout further so that, like HBox/VBoxLayout, you can specify alignments for each element then the layout will perform the alignment.
Technically you can align elements in a SpringGridLayout right now if you wrap each element in an HBox/VBoxLayout before adding it to SpringGridLayout, but ultimately letting SpringGrid do the alignment of it’s own child elements would be more efficient.