Does it make it easy for make transparency hair?
No, this is useful for textural re-arrangement only. For example, you combine two hairstyles and need to fit the uvs of both into the hair space. Instead of doing the same to every single hair texture manually, you re-arrange the uvs and re-bake textures. While this can also be done in 2.7x, now we have the ability to re-bake textures with transparency like normal maps, specular or diffuse.
That is easy to say but not much when you are using a hair with maybe 200+ polygons that overlap each other a lot.
You are describing an issue of different nature, unrelated to layering.
With transparency hair, there are two common problems:
1. Z-sorting. Happens when the rendering engine is not programmed to distinguish how far geometry is from the camera.
For example, when you put pink sunglasses on top of orange ones, you, human, understand that the pink lenses should be rendered first because they are the closest to the camera. Blender's rendering engines also understand because they are programmed to feel the depth. Ts4 rendering engines does not, is not programmed to, because it needs to handle many other tasks in milliseconds.
To make it understand, you can layer the lenses by separating and joining them in a specific order, so that the pink lenses come first.
Think of layers of hair as these lenses. You understand that the back of the hair should be rendered first in this case because it is the closest to the camera. The game does not understand it.
2. Z-fighting. It happens when faces overlap, for example, when the other side of the hair strand was created with no offset from the existing one. This is the issue you have in mind.
When you create a plane, it has only one side, facing one direction. On the other side it appears invisible. In Blender you can disable Backface culling to display the other side as well. In TS4 it is impossible to do this with CAS items. Again, would require more computing power, so was not added.
Think of each strand of this hair as a plane. They display textures only on one side but need to do it on both.
Impossible to make them double-sided in ts4, so the only solution is to duplicate them and flip directions.
If this step is done correctly ( the strand is duplicated, offset from the original and flipped), there will be no Z-fighting. If this is how it is created, it is also easy to remove in any version of blender ( select everything with A, remove doubles with Merge distance set to 0,002 or so, Recalculate normals, Remove doubles in uv editor as well).
However, it is possible that the author of the hair decides to decimated the other side, which will create clipping. If this is the case, you will not be able to remove doubles. Then you will need to delete the other side manually and it is easier to do in newer version of blender because it can shows them to you. So you just select one red face, press Ctrl L to select the whole thing, then delete it. It can be done in a couple of minutes.
This issue, Z-fighting, happens only when you convert other creator's hair to ts4.
If it is your own, then it would not be of any concern to you, if you will model the hair correctly:
- create the hair ( one-sided only)
- separate the parts that need transparency into another meshgroup;
- test in-game if you need to layer that new meshgroup. Smaller strands with transparency may conflict with each other, but it is much easier to solve this conflict with layering;
- only then add the other side. This should be done after layering, otherwise you'd be doing double-work
