Hmm. I don't think Planetside should try to test and sanction, as you suggest, plug-ins. Here are my reasons:
1) How can they actually control whether plug-ins get released or not? Unless you suggest a system where Terragen will only use plug-ins cryptographically signed by Planetside, I don't see any way to do this. Such a system would make development of plug-ins unnecessarily difficult, and so would discourage it--exactly what we (including Planetside) don't want!
2) Stability of plug-ins, you seem to suggest, might be dependent on what particular combination of plug-ins is installed and in use, and might also be dependent on system architecture, etc. Even in only a dozen plug-ins were ever released, there would literally be millions of possible combinations of plugins installed/in use for Planetside to check before giving the OK for that twelfth plug-in. Everybody at Planetside has better things to do than check whether particular combinations of plug-ins are stable--improving the core program itself, for example.
3) Some sort of community will doubtless spring up around plug-ins, and a "free marklet" like behavior will ensure that only stable plug-ins with reasonable UIs[1] are widely used.
4) Some plug-ins will be commercial; in these cases, the developers have a commercial interest in making sure that their plug-in will be widely used, and so making it stable and giving it a reasonable UI.
5) For plug-ins that are not commercial, there is always the draw of fame and glory (heh) for making a stable, usable, useful plug-in. Of course, if the developer was just scratching his own itch and could care less whether other people like his plug-in, but is just throwing it out there in case anybody does, this doesn't apply.
I can see Planetside potentially featuring certain plug-ins known to be stable on their website, but this would certainly not preclude other plug-ins being distributed. Over time, some sort of plug-in repository/index (or several...) will probably develop[2], and the stable and usable plug-ins will rise to the top, as it were. Other plug-ins will largely be ignored, and in time their functionality will mostly be duplicated by new stable and usable plug-ins.
[1] Reasonable does not necessarily exclude text-based. For example, there's no real reason to write GUIs for many simple utilities found on Linux and other similar systems--command line is just as easy, even for someone like me who was raised on Mac OS
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[2] If nobody else has done it in a couple years, I will.
