Kevin,
A few layouts would be nicer if GBS could be used, e.g.
Label Averyveryvery long control which goes a long way
Label checkbox Label checkbox Label checkbox
Label Someother normal control
etc etc
If I put the "checkbox" line into another sizer then the left most label doesn't line up nicely or not at all depending on the actual label or having them all on individual lines makes the form longer then needed.
Is support for GBS on your list? Or did you have some reasons not to support it? Besides that it might need a bit more user code when adding a control.
e.g. control.SetSizerProps({'pos': (0, 1), 'span': (1, 3)})
SetSizerType after controls were added would also probably also be problematic to support.
Werner