Well you've pointed out air being drawn in from the vent, the manufacturers seem to suggest gas break out. Just curios what harm it does. I skimmed the MI's which said wasn't required rather than mustn't be fitted.How much air do you think is in a cylinder
I'm not certain you'd know unless you did a back to back test, shower pumps aren't the quietest at the best of times.I've never seen the need to install one on any domestic pump installation. I've installed hundreds of pumps and never had an issue. Don't forget that manufacturers only reccomend them, if they was the be all and end all then they would be mandatory.