Squall is a quiet young man, and a lot of his dialogue is internal. When playing Final Fantasy VIII, I'm often struck by how deeply the game allows us to look inside Squall's head. No other Final Fantasy game places us quite so firmly in the protagonist's perspective.
This is out of necessity, of course; the story wouldn't work otherwise. Squall's emotional development is central to the plot of Final Fantasy VIII, but he's such a private person that it's hard to convey that development without giving us direct access to his internal world. Squall would give a very different impression if we couldn't hear his thoughts.
For a memorable example, think about how confused Squall's teammates must have been when he yelled, 'I'm not having anyone talk about me in past tense!' and ran out of the room. As the audience, we've seen his mental spiral leading up to that outburst. For his allies, it seems to come out of nowhere.
One of my favourite moments of insight into Squall's character is a brief thought that you can only see as the result of an optional choice. When Squall and his companions (Zell and Rinoa in my playthrough) are exploring the MD level of Balamb Garden, they come across an unstable-looking ladder. As Squall, you can choose to check it out yourself. If you choose to send someone else instead, though, Squall has this moment of internal conflict:
Squall: (I'll have someone else check it out.)
Squall: (Zell and Rinoa...)
Squall: (Zell...)
Squall: (Rinoa...)
Squall: I'll go take a look. You two wait here.
It's a lovely, subtle glimpse behind Squall's efforts to tell himself he doesn't care. When he thinks about his companions, and about potentially sending them into danger, he can't make himself do it. On the surface, nothing significant happens in this scene; Squall just comes across a ladder and says he'll take a look. But the fact that we can hear Squall's thoughts gives it an extra dimension.
If Square ever remade Final Fantasy VIII with voice acting, I wonder how they'd handle Squall's thoughts. They're such a huge part of Squall's character, but it could feel awkward to have a constant voiceover of his interior monologue.
Squall's so quiet, and he hides and represses so much. If the player can't hear his thoughts, the entire emotional core of Final Fantasy VIII is lost. Whether his thoughts are simply concealed or become spoken dialogue instead, he becomes a different character.
Maybe he'll just look at the screen and talk directly to the audience, like Malcolm in the Middle. It's a bad idea, but it would be extremely funny.