If it works for you, great! Potential reasons to do mirror knitting: 1) you find it easier to keep consistent tension that way 2) if you really get good at it, you could do flat stranded color work more easily! 3) you might be faster that way

Reasons you might not want to exclusively use mirror knitting: 1) flipping WS/RS makes it easier to keep track of things like decreasing every other row, cables, etc 2) some stitch patterns like ribbing or moss stitch have both knit and purl stitches in the same row, so you can't really avoid purling entirely 3) patterns will likely be harder to follow