Does this matter? Well, it does if you like real cakes and real buildings. The manufacturers of instant cake mixes, like the makers of instant plaster mixes, will doubtless say they are only supplying what their customers want. Nothing wrong with that, but the danger is that the customers want these products only because they have been brought up with them. They often don't realise that there is another way - a way that doesn't involve buying a packet with the instructions on.
So when a young plasterer - in his teens or twenties, say - is working in an old building with a patch of damaged plaster on the wall, the only material he knows how to use is gypsum plaster. So he mixes it up and spreads it over the offending area; bosh bosh, loadsamoney, as we all used to say. But the gypsum plaster behaves differently from the lime. It is harder and less flexible, so a crack will soon show up where the two materials meet. And if the plasterer makes the common mistake of using gypsum plaster on the inside of an external brick wall, something else will happen - after a while the plaster will get damp and the wallpaper or paint will start to discolour. This is because gypsum plasters dissolve when they get wet. Experienced plasterers know this, so they use sand and cement render on external walls; this stuff certainly won't dissolve, but it doesn't allow the wall to breathe either, and so can create other kinds of dampness problems.
Now, lime is one of the oldest building materials. It has been used successfully for thousands of years, and only in the last 30 years or so have we abandoned it. Unfortunately, it only takes one generation for knowledge to be lost; there are still a few plasterers in their sixties who were taught to use lime as apprentices, but few of them are passing the skills on. This is hardly surprising when kids are now told that the skill they most need is to be able to use the Internet. Very useful, I'm sure, but you'll never get a computer to plaster your ceiling.
So, if you are getting some plastering done on an old building, try getting your plasterer to do it in lime. Purists insist on using real slaked lime, but bagged hydrated lime from the builders' merchants will give a reasonable result, knocked upwith washed plastering sand - cover it up and leave it to stand for three days before using it.
The only tricky bit is getting the hair, but you can order this by telephone from IJP Building Conservation (01734 462697). The firm does horse, goat and cow hair for pounds 10 per kilo, and a kilo of hair will do about half a tonne of plaster.
Your plasterer will almost certainly want to add cement to the mix, because he will have been told that lime on its own won't work. You will just have to be firm here - explain that if lime worked for the Romans, then it will work for him too.