Will Rose....so often people ascribe a disproportionate 'value' to the skinners' contribution via submitted content and forget it is actually a true symbiotic relationship.
Certainly Wincustomize needs the content but it's important to remember that by and large, skinners also need the public exposure only sites such as Wincustomize can offer.
I know that were I to have a private site of my own I'd be hardly likely to have over a thousand 'reg'd users' accessing it at any one time, nor could I afford the associated band-width demands to do so.
Symbiotic relationships require contribution from both parties.  High-volume, popular skinners, yes, they DO provide the content, but consequently their skin downloads also contribute significantly to the running costs of the site which is largely financed by Stardock.
Now, if there's a requirement to be a subscriber to participate in remuneration it cannot be a 'bad thing' if in doing so you actually end up effectively refunded, and/or better still, come out ahead.
Generally people do support the site by more than 'just' their upload content but also through purchases, subscriptions, etc....and THAT is the 'good thing'....
Personally, I see no reason why I would not subscribe to a site which hosts my works for me, or financially support it in other ways as well, so I do.  I have around 150,000 downloads....dunno what that would cost specifically, but I'm betting Stardock paid for most of it, since a subscription, tee-shirts, CDs, etc can only go so far...