Even though using high quality parts undoubtedly benefits my customers, that's not the main reason I use them. It's mainly for my benefit.
Generally speaking, the lower quality parts have a much higher defect rate. For example low quality iPhone and iPad screens, may have a defect rate of about 25%. Which mean 75% of customers are happy and will likely not have any issues whatsoever. While the other 25% will encounter problems down the road somewhere. To have 25% of customers either coming back to have it fixed under warranty, or worse not come back out of sheer disgust does not make for a good business model. If I was to use poor quality cheap parts with a semi decent warranty in place, I would end up chasing my tail with excessive amounts of warranty repairs, which means doing all these repairs twice, or worse... excessive amounts of negative publicity. Some businesses will get around some of these problems by only offering a 30 day warranty.
So in using only the best quality parts available I can cut the defect rate down to 1-2%, which means I work on 99% of devices just once, and it keeps potential negative publicity down to virtually zero. All while being able to offer a 12 month warranty. This is the type of business I would prefer to run.
It may be a little more expensive for the customer, but the potential for problems are much MUCH lower when using the higher quality parts. And from a business standpoint, it's a no-brainer.