Pins are not just for holding pieces of cloth together. At least in Ankh-Morpork pins are something worth collecting. In this city over twenty-seven million pins are produced per year by the combined workshops called pinneries. Among them are specials like wax-headed, steels, or silver-headed. Lapel pins, or blazons, do not count since they are not true pins. Mere needles are looked down upon as just faux-pins with a hole knocked through one end. Nails cause strife: Dave refuses to have them in the shop, citing as his reason the fact that some collectors are young boys, thank you very much. The collectors of pins call themselves pinheads. A great number of magazines on pins exist with titles like Pins Monthly, Popular Needles, or Total Pins. The specialist shop to go to for all things acuphile is Dave's Pin Exchange, on Dolly Street. The collecting of pins has decreased greatly since the collecting of stamps has caught on. The details on pins are revealed in Going Postal
Sometimes pins have been discussed for more practical reasons. In Men at Arms, when Fred Colon finds a drawing pin stuck in his sandal, Lance-constable Cuddy advises him to keep it, as his cousin Gimick makes them, and they sell at five for a penny. This is not an interest in pins as such, more a generalised Dwarvish interest in good metalwork and value for money. As the old maxim goes: See a pin and pick it up and all day long you'll have a pin. Another major producer of pins is Josiah Doldrum's pinnery, of Cheap Street.
In I Shall Wear Midnight, a plausible reason is advanced as to why witches don't do the thing with the pins and the wax doll as much as they are popularly thought to do. The reason is not ethics or morality, it's simply because out in places like Lancre and the Chalk, pins are both hard to come by and bloody expensive, so you don't want to waste them by sticking them into an effigy and burying the dummy to rot and rust. Good pins don't come so easy. And they have at least two other uses - witness Granny Weatherwax and her multifunctional hatpins.
Pins feature on the British twenty pound note. This honours laissez-fare economist Adam Smith, who devised his theory on the division of labour (the first stage towards establishing a mass production line, and indirectly the inspiration for Karl Marx' theories about the alienation of labour in a capitalist society) after making observations in a pin factory. A reproduction of part of the pin-making process is part of the £20 note's design.
At first sight, Google offers pages and pages and pages concerning pin-collecting. Look a little further, however, and it's all about lapel pins and blazons. Searching on "acuphilia" just brings up hundreds, maybe thousands, of Pratchett-related references. Ah well...