How Neville navigates the challenges of living in isolation and manages to find a cure for the virus makes up for the rest of the story.
Suddenly, Neville is able to recognize the creatures' humanity, and he opens the glass partition to free the cured Darkseeker.
I'll have to look it up.
Later on, the seekers actually set a trap for Neville, using the same tactics and even the same rope snare that he used on them, demonstrating a certain level of intelligence and group thinking, and are seen using infected dogs that they've obviously domesticated and trained.
Related: Another big part of the I Am Legend book that's never been adapted faithfully is its ending, which sees Neville die by suicide rather than be executed for his apparent crimes.
I Am Legend Ending: Does Neville Find the Cure? The main criticism leveled at France Lawrence's film was the fact that the ending was so different from the one in the books to the point that it completely missed the point of Richard Matheson's story.