A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently and die gallantly. Specialization is for insects. - Robert Heinlein
Versatility is a curse; one dimensional people make all the money.- Jonathan Winters
I wonder, did Asimov ever ask Heinlein why a Jew would want to butcher a hog?
One always takes some risks writing about Heinlein. A sizeable cult of personality has grown up around RAH, its members being quick to pounce upon errors of omission and commission, and also ready to invoke Heinlein to bolster claims of their own political sagacity. To be fair, Heinlein did at least run for political office at one point (while still part of Upton Sinclair's Socialist movement), which gives him more practical political experience than most of his fans. On the other hand, running for office is not at all the same as governing.
Still, of the Big Three, Heinlein, for all his anti-specialization preaching, was a very specialized writer. Now it's unfair to compare anybody with Asimov in this regard. Okay, maybe Silverberg can pass the I-wrote-more-different-kinds-of-stuff-than-Isaac test, but, Isaac would then reply that he signed his own name. (Which wasn't always true, since he did write some juveniles under Paul French and he had a couple of minor "pulp pseudonyms" that never had books attached to them).
But even ruling out the comparison to Asimov, Heinlein had less range to him than Clarke, whose non-fiction was abundant and interesting to those without much interest in SF per se. Here Clarke's abiding interest in oceanography comes into play. Similar work from Heinlein would have required RAH write non-fiction about politics, naval topics, or non-SF engineering, and he just didn't do much of that.
Also, and I write this with much trepidation, when it came to science and engineering, Heinlein talked a better game than he played. We forgive him for the technical lapses, because he was writing crackling good yarns, but he'd often show basic gaps in his technical knowledge. I have the beginnings of an essay on "The Long Watch" enumerating the large errors in basic radiochemistry contained therein, and the description of the nuclear rocket drive in Rocket Ship Galileo demonstrates a significant misunderstanding of the idea of specific velocity in rockets. In The Rolling Stones he describes a rocket propulsion system as being "almost 100% efficient," which is a bizarre description of a form of transportation that puts the great majority of its energy into its exhaust, rather than its motion. It's not entirely clear that Heinlein ever got Special Relativity; despite using of the time dilation effect in Time for the Stars, he has one of the major characters baffled by a question about relativity that has a pretty trivial answer.
And so forth.
Heinlein's version of authorial versatility was essentially selling the same stories to a broader range of markets. He tired of the limitations of Astounding so he became a regular in The Saturday Evening Post. He wrote juveniles, then wrote off the end of the program with Starship Troopers. He wrote science fiction, and then when he wrote fantasy, it still read like his science fiction.
None of this is a bad thing. Limitations are not bad, and specializing is not a crime. Nor, for that matter, is it a crime for a specialist to advise his readers to specialize less and be more generalist in outlook. Versatility may not be the most efficient path to worldly success, but it has its rewards, and the specialist is more vulnerable to changes of fortune. If Heinlein had been a totally specialized naval officer, good for nothing else, then he'd have not been able to make the career jump that his tuberculosis mandated. Perhaps that is the source of his dicta about avoiding specializion.
Still, I am bound to note that Heinlein's list that begins this piece seems an awful lot like a list of things that Heinlein himself could do. There are so many things missing from the list. Play an instrument? Sing in an opera? Pilot an aircraft? Explore the Great Barrier Reef? Publish a scientific paper? Prove a theorem? Sketch a portrait? Calibrate a spectrometer? Blow a glass vase? Smith a wagon wheel? Spin and fire a pot. Catch a fish bare handed?
And when you ask me to program a computer, do you want that in Fortran, C, or Java?