There is certainly a great benefit that can be drawn from having the play's main character serve as the narrator. This, being that we can learn to appreciate the actions, thoughts and feelings of the main character in relation to the other characters of the play, and also that we may get to know them a little better by learning the main character's thoughts and feelings about them.
However, in this particular play we get to understand Tom better as a character because the stage directions are quite specific as to how he acts, and because he gets to speak with us at the end. This makes for a great chance to get an accurate character analysis of Tom.
Tom appears at the top of the alley. After each solemn boom of the bell in the tower, he shakes a little noise-maker or rattle as if to express the tiny spasm of man in contrast to the sustained power and dignity of the Almighty. This and the unsteadiness of his advance make it evident that he has been drinking.
This is an example of how, inThe Glass Menagerie, both Tom and the stage directions serve as the narrator and frame Tom's behavior.
For this reason, it is great to have Tom serve the two roles in the play. However, many of us may want to have the same thing happen with Laura and Amanda, just to find out exactly what is going through their minds. We may also want to know their own perspectives. Yet, we only get Tom's.
For example, when Tom tells us about Amanda and Laura from his perspective, he does so speaking as a brother who feels guilty for having left his sister at the mercy of their mother.
Then all at once my sister touches my shoulder. I turn around and look into her eyes ... Oh, Laura, Laura, I tried to leave you behind me, but I am more faithful than I intended to be ! I reach for a cigarette, I cross the street, I run into the movies or a bar, I buy a drink, I speak to the nearest stranger -anything that can blow your candles out !
Therefore, we can conclude that having Tom do the narrating in and out of the play certainly helps us connect to the main character in a huge way. However, the rest of the characters remain open to the audience to build its own opinion of them.