Before reading this thread, it never occurred to me how many different meanings it would hold for different people.
The saying isn't about a man only owning one gun...one gun being enough...and it isn't about only being able to shoot one gun well. It refers to a person who has taken the time to learn one platform so well that he can use it to it's ultimate potential. It takes some experience to really understand this, because at lower levels of skill, many guns can be used at the same level.
I happen to shoot several different platforms, because students make different choices and it is my obligation to be able to shoot them all well enough to teach it's use. But I made this choice knowing that I was compromising really being able the get th most out of any one platform:
1. my reset will always be longer than needed...because I don't want to short stroke the reset.
2. my prep of the trigger for the next shoot will be less than optimal...because my finger won't know if it takes 4lbs or 4.5lbs to take it to the edge of release.
3. my seeing the aligned sights return to target will always be just a hair slower...because they will look different than I have been training to see.
4. a longer shot will take more conscious thought because I'll have to figure the holdover for extended ranges.
If you aren't shooting at this level, the saying will have much less meaning. It is the difference between being able to shoot at a rate of 4 shots per second and 6 shots per second