Ok, sure, it's not always true. I shouldn't have used the word "never". But I think that in general it's best to go it alone. Often you end up with two people arguing over who does what work, and over dividing money.
However, I can totally see working with other growers in your area to make sure no one undercuts anyone else too much, or to collaborate on who will grow what so we're not all growing the same thing. And it it nice to have other people to take care of things if you have to go out of town or something like that.