it kills it dead, makes it weaker and the 2 times i have seen area's fail it sheared off the background. No i cant prove this was the reason but it was area's that the spread in question had a habit of using his clean out water for the first coat and sometimes second. on his other walls the surface was dusty and weak. also if you had a problem and BG came down, even if there was a problem with a batch they would get out of it because you have not mixed the plaster to there guide lines. it clearly states use clean water. Its also not a good idea to promote pratices like this on an open forum which someone who is learning will copy. I don't care what you do on your jobs but i wouldn't and any spread who i found doing it on my jobs would be off and 50% retention held until i was happy that there would not be any problems. You might say this is unfair but after he has gone and spent the money, who ha to pay to put it right if it does fail, ME.