Still the best thing I believe, if every individual trade let the other traders do their work for various reasons. First, I don't believe in myth as one trader can do other trades on professional level. On the other hand, it's support the buisiness if one let others to the table and effectively give them work in case they are reliable, as on the long term it's pays off for everybody as traders support each other and recommend each other which brings more client and businesses than try to do everything by themselves. You can still provide full service by involve your partners you trust and you can focus on your profession in case larger projects especially if you re popular among contractors