This question will polarize opinion. Having worked in the industry and having many hundreds of people, back in the day, bring me cartridges saying they needed a new 'needle' and having taken their old styli home (after they consigned it to the trash can in the shop), and putting them under high powered magnification after cleaning, made me realize, most if it was utter and complete BS. Marketing in it purest to sell replacements.
People around here will wax lyrical about the enormous differences a new stylus makes. They will tell you how many hours before it 'must' be replaced. Most of them don't own a decent microscope, they haven't looked at their diamond under either digital or optical high powered magnification, illuminated effectively and/or made measurements using test records and proper test gear.
Dragging the hardest substance known to man, through a piece of plastic, is a walk in the park for a diamond. It can happily do that indefinitely.
And to anyone who wants to show some reflective light spots on a facet under a microscope- show me the actual before and after frequency response plot for that stylus. A shiny bit means absolutely nothing.