When your game was released, the compatibility is very important. So we prefer to use ETC1 texture format to compress texture on Android. ETC1 is part of OpenGL ES 2.0 and is supported by all OpenGL ES 2.0 GPUs. But ETC1 does not support alpha channel.

ETC1+Alpha is a solution to use alpha with ETC1 format texture. The alpha channel is delivered as a second packed texture. Both textures are then combined in the shader. ETC1+Alpha provides many shader to support 2DToolkit, NGUI and built-in shaders.