The problem stems from the png format not storing an alpha channel, it only stores transparency. This means that if an area of the image is 100% transparent, then the png format is free to not store any information about the colour of those pixels. This can cause problems for game textures for a number of reasons. For example, the alpha channel is often used to store information other than transparency (specular hardness for example, or any other useful information). It can also be a problem when creating mip-maps for textures (scaled-down prefiltered versions of the texture), as it can cause nasty halos around the edges of sprites.
The tga format stores an explicit alpha channel, so it doesn't have this problem (it is also extremely simple to implement, which is another reason game developers like it).
So on behalf of game developers everywhere: please add support for exporting to tga!
