More detailed textures have their own advantages ...
But a smart decision about the textures format, especially chosing a common format, was not yet the top priority of the development team - there are so many different formats at the moment, and I even found several graphics which were made poorly, not using the features of specific formats correctly - e.g. different kinds of transparency definitions, wrong alpha channel distributions (see the "distant-tree" image borders), even different colorkey values for similar icons sometimes (compare the crystal icons with others).
Somewhere I read that the developers planned to introduce a common texture format. But I could not remember its name, it was too uncommon, probably quite proprietary. I hope it is related to some GPU internal texture-compression techniques (like S3TC).