As hard as it can be to get the correct stitch count in a gauge swatch, row count is harder. I don't know why, someone else undoubtedly does. I've gotten stitch count and but not the row count more than once. Stitches/inch are certainly higher priority than rows/inch. If a pattern gives measurements (it might say to work 4" from the cast on edge) row gauge is less important. I much prefer patterns to give a measurement to work with.
ETA Some patterns say to work x rows and the length is based on the row count. I really dislike this in armholes and sleeves, a measurement would be much more accurate.
Happiness is catching that dropped stitch while it's still fixable..