I agree... A skin with only one panel must be specially designed for it.
However, XMPlay is changing. This has to be the biggest GUI change for a while now, and I am eager to see how it turns out. So I guess this means that each panel (besides Output Options) will no longer be customized with each skin. I wonder if a new button will be added to bring up this window? And as Jacer says, maybe other extra buttons could be added to the main panel like the 'Skin author' (for customizable "Designer Signatures"). What the heck, with only three panels now why not mix all the mask palette indexes into one main list? That could have benefits... for example, you could make those close / minimize buttons work on the playlist of the Nullsofted skin!
Then the old skins are somewhat worthless now. Let's round up all the skin authors, they can and should be updated to XMPlay 2.8 (although the final decision is that of the skin author). Like RahDick says, the skins were designed with multiple panels in mind, and like Kynes says, some will look weird with the panels being missing. These need to be corrected so the skin will look good and have any extra buttons that XMPlay 2.8 may use.
The skin creator (me for example) has put a lot of work into the panels - and why remove them if they still can be used?
But they can't be used in 2.8, only in previous versions. All 2.8 users should be able to have versions of the skin without extra useless bitmaps. Even better, have updated versions of the old skins so they look and work well in 2.8. As for compatibility with older versions, maybe keep an older version on the Support Site, or as Ralesk says, an old archive page. But I guess if the skin author doesn't want two versions like this, they could choose what happens to their own skin.
When I think of some skins, like X-RD2506, I realise a lot of work has gone into the other panels, and now will not be used in 2.8 onwards. This is unfortunate, but some would say it's a form of progress. Still, every XMPlay skinner has put some time and effort into those other panels, so I wonder if there's some way they could still be used in 2.8. Maybe use BoggyB's suggestion, just don't use them as standard...