Ephialtes the hunchback. (SPOILER ALERT)

Hindsight is 20/20? Leonidas probably didn't think that Ephialtes would go so far as to betray his king and his country because of some truth bombs. In fact, Leonidas' tone is quite compassionate and he compliments Ephialtes on the strength of his arm; Leonidas only points out the truth: Ephialtes is physically incapable of joining the rank.

Now, given that he had 300 Spartans and some ~1,000 Thespians, he probably should've just let Ephialtes do something. He could have stood at the back, thrown spears, or - y'know - guarded the goat path as a scout... So, yes, Leonidas should have had him do something.

Is there a case against? Yes. Including Ephialtes might have made his other men think Leonidas was "soft" or desperate. It might have broken the camaraderie if they felt an "outsider" was there. But that's a small case. Probably wouldn't have hurt them.

A little push-back on Leonidas being a dick: imprudent? Maybe, although again, it's hard to fault him for not anticipating such a betrayal. But either way, I don't think he was a dick. In the world of 300, Ephialtes is lucky to be alive. By Spartan society, he was doomed to be cast on rocks as an infant. The fact that Leonidas responds in a compassionate tone and isn't dismissive from the outset speaks to his strength of character.

Yea, I agree, Leonidas should’ve put him in the rear or something or given him a medic detail like duty, although things would’ve most likely still gone to pot, the secret goat trail Ephialtes gave up to Xeres certainly wouldn’t have been the reason, but they were so outnumbered it was only a matter of time.
Good question though, every time I watch 300 I wonder the same thing. Why be a dick?

The hunchback wanted to be a part of a phalanx. He would've been insulted and rejected the moment Leonidas said join the rear guard. The result would've been the same.

The whole idea about the Spartans was that they were outspoken and economical with words.