"I have a friend (or brother-in-law, or Uncle Louie, or insert friend here) who is willing to do the tile work in my new home for a discounted rate. I'd like to use him and save some money." I can't tell you how often I have heard this from people. My response is always that I'm happy to have him bid on the work and if he can provide us with insurance, licensing, acceptable references and sign our subcontractor agreement, we'll consider using him.
In reality, I've learned the hard way that it's not a great idea to allow a homeowner's friends or family to do work on a home that I'm constructing. Frequently it turns out to be a disaster for both myself and homeowner. The most common issues are delays in the start and completion of the work. When I communicate with our subcontractor and we agree that the tile work will be completed in 3 days and then it takes 3 weeks, that ends up being additional interest expense and / or rent for the homeowner due to the extended time to complete the home. Other common issues are quality of the work and warranty support after the home is completed.
One of my more entertaining builder colleagues likes to use an analogy when he is asked this same question. Here's what he says. When you visit a dentist, you expect him or her to be trained and equipped to do the job. You would never think of bringing in your own tools and materials, handing them to the dentist, and asking if you could save money on a filling or crown because your friend or relative is in the dental supply business. Similarly, you wouldn't take a steak to a restaurant and ask them to cook it for you so you save a few dollars on a meal. I like this approach, since it really isn't any different in homebuilding. It really makes senses to let an experienced builder do what he does best.
Very simply, I've learned that at the very least using friends as subcontractors will disrupt the smooth flow of work and communication. At its worst, using friends as subcontractors will produce a conflict and strain on a personal relationship.