This topic I would choose to disagree on because after-market auto parts may not be bad as you think. In some cases, after-market parts can actually be superior compared to your OEM parts. This is evident in parts such as after-market brake pads, after-market spark plugs (iridium plugs etc.), after-market air filters and even spark plug wires. Only a few are mentioned here, since there’s quite a bit.
When maintaining your car, you should put quality parts into it, so that you would essentially reduce your maintenance intervals and in the long run, prolong the life of your car. For example a manufacturer of a particular type of car, may recommend that you use a 10W30 non-synthetic engine oil when performing an oil change but if you use a 10W30 fully synthetic oil such as Royal Purple in your car, it would be better than what the manufacturer recommended. The Royal Purple oil would have detergents that would clean the engine so that when you change the oil again, it would come out dirty but would get cleaner with future oil changes. In addition a fully synthetic oil is far superior to a non-synthetic oil in that synthetics provide better lubrication due to more even molecules in the oil (these oils are made in a lab) whereas the non-synthetic oils are made in a refinery from crude oil.
I personally use after-market street performance brake pads (Hawk performance pads) on my sport car and they make a huge difference compared to the OEM brake pad. These after-market pads tend to produce less dust and are virtually noise free (depends on brand). They also provide greater stopping power! There are other similar brands such as EBC brake pads or Akibono brake pads etc.
In addition after-market spark plugs are usually more durable compared to OEM spark plugs. For example some after market plugs have iridium tips which are more durable and provide bigger sparks and also utilize less current to do so. They usually give a small boost in performance.
Hence I would never make the statement “You should never use after-market auto parts when maintaining or fixing your car”.