Mike
If you were correct (don't worry, you're not:) and FS is searching in all these locations, then let's look at a couple of examples:
a) If both RealNZwgtn and RealNZmarlborough have the same texture file, what would happen if you removed one of them? Would it still display the texture in both sceneries?
b) If I removed the texture from both sceneries, and put it in the Addon Scenery\texture folder, would it display?
(You may wish to try these experiments, but I'll save you the trouble -- the answer to both will be no.)
But if I removed the texture from both, and put it in the Flight Simulator 9\texture folder, it would find it, albeit AFTER searching the appropriate local texture folder first. HOWEVER, if I left it in both, and also copied it to Flight Simulator 9\texture, then it WILL DISPLAY THE TEXTURE FROM THE LOCAL FOLDER, AND WON'T EVEN LOOK AT FLIGHT SIMULATOR 9\TEXTURE. (again, this is easy enough to prove -- or disprove if you like.)
The Addon Scenery folder is simply another scenery folder, it has no special powers, and is not in any texture search path. (However the Flight Simulator 9\Texture folder is.)
There is a lot of confusion about the Addon Scenery folder. This was originally added to allow scenery to be pre-activated after installation. Because the Addon Scenery folder was already activated, you could drop your scenery into it without having to activate it. This, of course, was back in the days when scenery was made up of a single BGL, and used default textures. It really has no relevance with today's huge sceneries. It is, however, used for RWY12-type scenery, again because it allows for drag 'n drop installation. Of course you will get into a mess if you try and have two different textures with the same name here -- they simply can't exist in the same folder, and one will be over-written by the other, meaning that one set of objects will display incorrectly.
Between Wellington and Marlborough, there are more than 20,000 texture files. You expect the sim to sort these textures instantly, so that it knows exactly what is there and how to use them, but somehow it gets confused and stuttery over a handful of similarly named textures?
You are right that it is generally better to have just one texture rather than four, but there are very good reasons why I do it this way. First, it makes for a much easier, troublefree install, and believe me, people appreciate trouble-free installs. Secondly, back to my original point:), a reusable texture needs to go in Flight Simulator 9\texture, but if I have a library BGL in my local scenery folder, the sim will first look in the local texture folder, and only if it doesn't find anything will it look in the global texture folder -- certainly not as quick as my way, eh?
EDIT: OMG I just found out why FSX performs so badly! There are 135 different instances of the texture called fresnel_ramp.dds! That's meant to be funny, and should be taken as such. However I also find
this funny, which may explain a lot...