Except you can apply that logic to other games because other games don't charge extra for more characters.If you only chose one job in FFXI then what your saying is true. Fortunately (or not depending) we can walk to our pet moogle and *BAM* instantly respec / change jobs. This ends up exasperating the problem by enabling high end players to always be capable of doing the "near perfect" setup. Why would anyone bring a non-perfect job when they can change to the perfect job or be replaced by someone else. The only time this is no longer an issue is when the jobs in question are so similar that their functionally identical in 90% of cases. DRK vs WAR for example.
"Why take x person when we can take y person?"
"Why take x character when you can hop onto your y toon instead?"
In other games it's pretty normal for people to play multiple characters in compensation for the fact that a job change system doesn't exist. The same logic would apply, but it doesn't because there generally isn't that gigantic gap between classes like this game has.
And, WAR vs DRK, as you said, are pretty much functionally identical when it comes down to just filling in a holy trinity role (in this case damage). They still have unique function such as fell cleave, multiple high skill weapons VS absorb spells and twilight scythe etc. This should be the philosophy when designing other Damage jobs. Make them capable of dealing damage, and then add unique things to their role to make sure not every job is the same. It's precisely because that there's only so many ways to crack the pinata that it's important to try to make it so that everyone can function as the bat in about the same manner when that's the main attraction. I can use either a golf club or a bat of the same length to crack the pinata about the same, but when i go play baseball or golf obviously I"ll choose one over the other.