A well-read Scottish MSP, Alex Johnstone, has tabled a motion to the Scottish parliament to do a kind of Richard III-style rehabilitation job on Macbeth. Shakespeare's version? Lies, all lies. If I read him correctly, Mr Johnstone goes so far as to fantasise a Macbeth theme park where the maligned monarch fell in battle.

It's not recorded whether Mr Johnstone, like the admirable Philippa Langley, has experienced a "trembling of the knees" treading on the monarch's grave on Iona. And if Macbeth is a candidate for rehabilitation who next? Rasputin (much maligned). Nero? He's had rather a bad press for the past 2,000 or so years.

In fact Richard III had more going for him than most stage villains when it comes to historical rehab. He's disabled (amazingly, the skeleton proves it down to the last vertebra). When Anthony Sher did Richard III he came on with so many crutches he looked more like a walking prosthetics store than a monarch.

I fully expect, after what we've recently discovered, to live to see a modern dress production of Richard III with the hero rolling on stage ("now is the winter, etc etc") in a motorised invalid carriage climaxing, at Bosworth, with the agonised "A go-kart! A go-kart! My kingdom for a go-kart".

And there's that picture of Richard in the National Gallery, which – again amazing – is a spitting image of the recent forensic-lab reconstruction. Josephine Tey wrote a brilliant novel about what is to be deduced from that NG portrait, Daughter of Time (1951). Her detective hero, Alan Grant, a brilliant "reader of faces", takes one look at it and concludes it is not the face of a mass murderer, but "a candidate for a gastric ulcer". And, one can now add, a chronic back-ache.

Back in the 11th century, royal portraiture hadn't made the advances so brilliantly displayed in Paul Emsley's recent depiction of the Duchess of Cambridge (no, I'm not serious). What did Macbeth look like? We'll have to dig up the skeleton and send the skull to the lab if we really want to know. I don't think Johnstone would want that.

But does it matter? If you want history, go to the library or to Professor Google. If you want drama, go to Shakespeare. It's not literature's best kept secret that the playwright was dreadfully unfair in his depiction of the blood-drenched tyrant. He maligned Macbeth in order to butter up his patron King James. You do that kind of thing when you're the resident playwright of the King's Players.

As a coolly informative BBC CV tells us, just to take all the fun out of the play, history was boringly different from what Shakespeare offers his audience. "Macbeth," so it says, "was a king of the Scots whose rule was marked by efficient government and the promotion of Christianity." Point taken, Mr Patten. But a play devoted to the celebration of Scottish efficient governance (a subject dear to Mr Johnstone and his colleagues) would not, I suspect, keep bums on seats till the fifth act.

I suspect however, that the motion to be brought before the Scottish parliament has subtler motives than simply putting the record straight. In the play, the English are Scotland's friends in need. Malcolm recovers the throne with the aid of a Sassenach army. Macbeth, viewed objectively, is a persuasive plea for the union of the two kingdoms (a sentiment which I, with as Scottish name as Mac Bethad mac Findláich, wholeheartedly share). It was for King James VI and I, first sovereign of Scotland and England, that Shakespeare probably wrote his play (in huge haste, scholars theorise, for a state visit by the King of Denmark).

There are then, two reasons for continuing to honour Shakespeare's version: it's great drama and good politics. It's horrible history, but so what?