Shatner has openly admitted that he was sometimes dismissive of his co-stars and inconsiderate of their feelings. He has lessened his ego over the years, and has even made some attempts to reconcile. He and James Doohan buried the hatchet before Doohan's death, for example.
However, I think things have also been inflated over the years. As has been pointed out here, TOS was not
an ensemble show. Ensemble shows, as we know them today, were virtually non-existent on TV back then. The formula was that you had one or two main stars, a guest star of the week, and then some supporting players. That's the formula TOS followed.
The supporting characters were important inasmuch as someone needed to pilot the ship or open the hailing frequencies, but their character development was not important. Can you think of any episode of TOS where you could not have removed Sulu or Chekov or Uhura and substituted a nameless ensign? In fact, TOS did this frequently when a particular actor was not available. Now, imagine trying to swap out Kirk or Spock or McCoy for a different character. Wouldn't work.
Once TOS became a cult sensation in the 1970's, fans began to elevate the supporting actors to something of legendary status, and those actors began to see themselves as of significant importance to the success of TOS. But, at the time, they were just supporting players. That's not being cruel. That's just the way television was at the time.