Of course a single pellet, or a mix,
can be nutritionally complete; that doesn't mean that they
are, manufacturers' claims notwithstanding.
Add to that the fact that each pellet has a different recipe and composition, throw in the fact that each fish species will have slightly or greatly differing nutritional requirements, and it all adds up to a strong case for playing the odds to your advantage by offering a variety. It simply doesn't stand to reason that a single recipe will be perfect for every fish, or every cichlid, or every catfish, or every...
anything. Feeding a variety is also useful in case the food you prefer is for some reason unavailable; having others on hand that are known to be acceptable could be a useful feature.
There's another factor as well, a human factor. I and many others simply feel that a living creature deserves some thought given to its quality of life, whether that quality difference exists in reality or simply in the keeper's mind. I wouldn't feed my kids a "nutritionally perfect" pellet if one existed for people; I don't feed my dog a single commercial "nutritionally perfect" dog food either (although I'm sure that such a product probably exists). Even if I believed that the research into fish nutrition were as intensive and exhaustive as that which goes into human or canine diets (and I very much doubt that), feeding a living creature a single monotonous diet "feels wrong" to me. It's not a motor that can use the same brand of gasoline and lubricating oil indefinitely without complaint. A living thing deserves some amount of empathy in its maintenance. So I am going to look for these other alternative pellets, and will also continue feeding frozen whole krill, earthworms, mayflies and other foods as well.
We keep them because they make us happy; keeping mine this way makes me happy.