A commonplace book is what a provident poet cannot subsist without, for this proverbial reason, that ‘great wits have short memories:' and whereas, on the other hand, poets, being liars by profession, ought to have good memories; to reconcile these, a book of this sort, is in the nature of a supplemental memory, or a record of what occurs remarkable in every day’s reading or conversation. There you enter not only your own original thoughts, (which, a hundred to one, are few and insignificant) but such of other men as you think fit to make your own, by entering them there.The passage is quoted in many places as positive advice. It should be clear, however, that Swift is deeply ironic, if not sarcastic or downright satirical. The "advice" is just as little serious as the observation that "it may be necessary for your ease, and better distillation of wit, to put on your worst clothes, and the worse the better; for an author, like a limbeck, will yield the better for having a rag about him." A commonplace book is common or trivial for Swift, and so will be the writer who relies on the "few and insignificant" original thoughts likely to be contained in it.
He would have chuckled at the people who take his mockery seriously as advice to be followed. I am sure he would have thought that they prove the very point he wanted to make about modern letters. I am sure that almost everyone who quotes the passage has not read it in context.
I tend to agree with Swift that commonplace books have had their day, and that they represent a way of supplementing memory that is best forgotten.