Not only do they not matter to the plot, they don't matter by design. Rather than allowing you to change the path of the story, the devs used several of the choices to affect your mindset and to make statements about aspects of video game storytelling. Eg, just in one of them...
Choosing to save the civilians vs. Gould. This "choice" has literally no impact on anything in the game. Pick the civvies, Gould dies. Pick Gould... Gould dies anyway. This choice is supposed to illustrate incomplete information. In a game like Mass Effect, when it presents you with a choice, you can be reasonably certain that the options the game presents are... for lack of a better word, not lying to you. Whatever the text says, that thing will happen if you pick it.
With the Gould "choice," Gould is already dying before you make your decision. Trying to save him is futile. But Walker doesn't know that, so you don't know that. (Also, the civvies have a few days tops to live thanks to the later story developments, and that's only assuming they don't go straight to the camp you drop white phosphorus on.) Also this ties in thematically with the WP scene itself and the Lugo/Walker "There's always a choice"/"No, there's really not."
They do have an impact on your mindset though. That was a huge part of what made the game such an emotional roller coaster for me. In one, I rejected the binary and felt like I'd scored a victory, getting back on the track to doing what was "right." In another, I coldly meted out what I thought to be justice and took grim satisfaction in it. In another, I completely gave up hope.
