An update...
W. Marshall Leach's paper was first presented to journals in 1994 but ultimately was never accepted. It remained available only on the Internet. However, in 2011 a response to it was published in the International Journal of Electronics by Robert I. Damper:
Can dependent sources be suppressed in electrical circuit theory?
https://eprints.soton.ac.uk/271202/1/superposition.pdfIt affirms the conclusions of Leach's reasoning providing perhaps a slightly more robust proof of the technique.