Which is preferred? Are both technically correct?
1. Mike scored 40 points; John, 17; and Victor, 2. (Is the added punctuation here [ie the semicolons and extra commas] unneeded?)
2. Mike scored 40 points, John 17, and Victor 2.
Yes, both are technically correct.
The second is preferred, as it results in a cleaner sentence less cluttered with punctuation.
(In general, avoid pauses if you can. Commas, such as the ones in this example, are only necessary if the sentence would still be meaningful with their phrase omitted.)
(Avoid pauses if you can. Commas are only necessary if the sentence would still be meaningful with their phrase omitted.)
I would say that the first option is preferred. The first option is a simple usage of ellipsis, i.e. using a comma to replace an omitted word (in that case it would be replacing scored).
I'm not convinced that the second example is a good idea because, depending on context, it could be interpreted incorrectly by the reader. I will reiterate that this depends heavily on context, but it could be mistaken to be referring to the Bible verse (John 17), or if the context was a police communication it could be referring to the radio alphabet, etc.