Consider the following case: a form in a panel of XWiki, with inputs and buttons.
In the flamingo skin we'd get the result in the second screenshot while in colibri the result would be the one in the first screenshot.
In the case of Flamingo, what is bothering is that the input does not fit in the panel. This can be problematic for existing applications which have panels and/or existing code.
In addition, this depends on the size of the screen (on a bigger screen the input would fit).
Now, if we put the class xform to the forms in the panels, it also fits in Flamingo, and this could be a solution for existing apps/code, to change the panels and turn forms into proper XWiki forms (with class xform), but a solution would be that Flamingo takes care of it, like Colibri did.