Two licences. Either you use the GPL licence with it, and according the GPL all part of the software MUST but under GPL compatible licence. Either you use the commercial licence and your software may have proprietary content.
If you read my post before, you'd see that all parts of the code that's integrated need to be under GPL. That doesn't count for art and data in use.
Plus, if you read the their licences page more carefully you'd see that there is no mentioning of having art and data open - or any other content except code for that matter. It is just a standard GPL limitation that allows you to use any content whatsoever with the code as long as your modifications on the code are in complience with GPL.
What Nevrax is doing by that is allowing FOSS projects and development to exist on one hand, while also allowing against a license fee proprietary companies to use the same NeL code and tools as the FOSS projects can, but with added limited support and the license that they don't have to publish their modifications to the code under GPL. Imagine Bioforge being interested in NeL - they probably wouldn't (at least Atari wouldn't!) want to open their source code because of the competition on the market.
Irrelevant, Firefox is not released under GPL.
True, it's not under GPL ...but it's a good example of plugins though.
Or, if you'd rather have Konqueror (the GPL'd KDE browser) for example - it can also use java, macromedia flash and other plugins. Or Xine or Mplayer that you can use with win32codecs, DivX and all the rest of the proprietary non-GPL-compatible gang. The important part is that plugins and codecs are
not a part of the code.
That's my point. The raw GPL licence don't allow to release other games using the PS engine with proprietary content as PS does currently. This is greatly reducing the potientiality of the engine, and furthermore, it's unfair for other developpers. The licence may be amended to allow exceptions (as it's the case for GPL fonts for example) or another licence may be added (like NEL but it could be free). In the current state, the PS licence set is inconsistent and this may cause trouble.
Yes, it does! GPL has no restriction whatsoever to disable GPL code being used together with non-GPL-compliant content. Do you think you breach the GPL everytime you load a non-GPL wallpaper on your Linux desktop? What about when you open up a document in OpenOffice.org or KOffice that you didn't publish under GPL (whatever you make is under your exclusive and complete rights automatically, untill/unless you state otherwise!)? Draw pictures in the GIMP or Krita? Or even view a DVD in Xine?? Or even more obscurely - watching non-GPL content on the internet with your GPL'd browser or read e-mails with your GPL'd e-mail client?
With that said, I think it's obvious that the PlaneShift client code is very well able to use the PlaneShift code in order to play. Also you're free to use the code - but not the art and data. What good is the code without art and data? Well ...what good was to anyone that id' GPL'd all their engines when the next game came out? Just search happypenguin.org for FPS that are based on the quake or doom engines

It's pointless to blindly defend the GPL as the best licence for everything.
That I agree with

But it
is the best license to make sure the code written is kept open and free.
On the side note: GPL as an idea is a child of Richard Stallman, but as a license it's Eben Moglen's work.