Beneficial for what? Sufficient for what?

If the important thing is maintaing external appearance, then secondary glazing can be sufficiently discrete not to detract from that appearance. Gaps beyond around 20 mm allow convection which significantly reduce the thermal resistance of the window, though may improve sound insulation.

In a building whose interior is of special historic significance, the internal appearance of secondary glazing may be unacceptable. In such a case the use of other energy saving measures may be given a higher priority.

Draught proofing is very much more important than double glazing. Very often replacement windows don't even do this so the huge cost is literally blown away. If for aesthetic reasons double glazed units cannot be used then just sort out the draughts and you will be OK. New windows take a very long time to pay back anyway.

Even if you do get the outside done you have a long way to go yet. Under the sill? and then from the cavity into the room or into the frame and out again into the room easy with hollow frames. In through the glazing seals or drainage holes etc.

Then there are the dreaded trickle vents --- can of worms ?? a whole bucket full !!

You must know jolly well Biff that it's easy to arrange deliberate ventilation, just how you like it, what's difficult is to get rid of perpetual unwanted leakage.

Ken, can you make a new topic of this - with compulsory HIPs/Energy Ratings coming in in July we need to know how it's going to work, whether it bears any resemblance to reality etc.