The problem with sealing bare plaster with PVA is it does not allow the first coat of V/Matt emulsion to sink-in-to the PVA sealed plaster (as it should to enable strong adhesion). This can create many problems when someone decides to hang a wallcovering over it, i.e. when someone decides to strip the wallpaper it will tear and rip the first coat of emulsion that has been applied over PVA. This causes for a longerterm problem, with easily damaged painted surface only apparent once stripped. This is why your emulsion appears to go on like a dream over PVA but there are longerterm implications for this practice. Same when these DIY Shows etc explain that you PVA over bare brickwork etc to seal the surface before painting. This causes the paint to be more prone to flaking and loss of adhesion but is only apparent longerterm. Also, PVA can cause small blistering of emulsions.