Only the strong survive
One problem is that this coating is not 100% effective. It was written elsewhere that it "only" kills 99.9% of microbes.
We have to assume, therefore, that the 0.1% that survive will be the strongest, most potent and therefore most dangerous ones.
While the wimpiest ones might give people an easily fought-off dose of whatever (thereby priming their immune system), leaving behind only the "schwarzenegger" variants, anything that a person does catch from an unhygienic keyboard (or telephone handset) will be worse for them.