Conjugate Symmetry of K-Space
The downside is a reduction in the signal-to-noise ratio (SNR) by a factor of √2.
If we reuse data, we are reusing the same noise contamination of the data as well. If we actually acquire the data, we'd still have noise contamination but it would be different because the noise is random.
The conjugate symmetry of k-space is as follows. To get the complex conjugate of a k-space data point, simply change the sign of the imaginary part of the complex number. To put it in the correct part of k-space, perform a reflection in both axes (equivalent to a 180° rotation).
See also the partial Fourier page of the K-Space Tool.