Are semi-colons really necessary, asks a correspondent?
Well it all depends on what she means by necessary its perfectly possible to have a long piece of writing with no punctuation at all and it still makes sense with very little if any ambiguity people arent used to it of course so it isnt particularly easy to read
And so on. James Joyce did it better (at the end of Ulysses). Punctuation gives us two kinds of information: it helps us see the grammatical structure of a sentence, and (the older use) it gives us a clue about how to read text aloud.
The semi-colon first appears in Europe at the end of the 15th century, and (according to M B Parkes) 'seems to have been a deliberate invention designed to fulfil a particular need'. Aldus Manutius explains it as a sort of compromise: the semi-circle (i.e. comma), he says, doesn't give a long enough pause, whereas the double point (i.e. the colon) slows up the speech too much. It rapidly spread in Europe during the 16th century, and is found in English from the 1530s. It took much longer to become widely accepted, probably because of uncertainty about its overlap with the function of the colon. Henry Denham was said to be 'the first to use the semicolon with propriety' (in 1579). It was well established by Ben Jonson's time, but typesetters varied enormously in their use of it. Semi-colons appear on some pages of Hamlet in Shakespeare's First Folio and not on others.
Today, the semi-colon has a clear grammatical (rather than a phonetic function), allowing writers to mark a level of grammatical organization higher than the comma. Its function is to coordinate. Semantically it closely resembles 'and':
John drank tea; Mary drank juice.
It is therefore used to link units at the level of clause or phrase - even allowing single-word units, when some of the other units contain a comma. The example I found for Making Sense of Grammar illustrates the value of the semi-colon:
The menu offered us juice; a boiled, fried, or poached egg; toast; and tea or coffee.
Replacing the semi-colons by commas makes this much more difficult to read:
The menu offered us juice, a boiled, fried, or poached egg, toast, and tea or coffee.
So yes, it's a useful mark, I think.