Seems like quite flat areas have gradient issues. This is because the associated vertex normals differ from each other (are pointing in different directions). These gradient issues can cause artifacts, banding, larger file sizes, harder to overlay other details on, etc.
Kodde: DXT compression is a fixed size, a 24bit compressed image is always going to be 1/4th the size of an uncompressed TGA. I see what you're saying about overlaying etc, however i dont know that i would often do it the way you mention, as even if my lowpoly normals match the high very well, i'm likely to have some areas…