To be honest...I've never played Battlefield 3..but the only thing I can ask is during the gameplay, are there times when it would make sense for him to talk? (like, someone talking to him or something thereof)
If there are times like that...then yeah..I don't get why they wouldn't go ahead and use the resource they have, even if it'd be for just a few lines
Well, that's alright if you haven't played Battlefield 3. This problem applies to other first person shooters these days.
With my example, there are many times the main character should talk, but doesn't. For example, in one mission (called "Night Shift"), two US Marines named Blackburn (the main character in the mission) and Campo (the squad leader) are regrouping with the rest of the US Marines near a mall in Tehran, Iran (there's a war in Iran in the game). Campo starts to talk about how their platoon leader (Cole) is a "gloryhound" who wants to be promoted to Major even though he's been Captain for a few years. Blackburn doesn't provide any input as to whether he agrees or disagrees with Campo on the matter, which makes him less interesting to know about.
One more example would be in the mission "Kaffarov", where Blackburn runs into a Russian secret agent named Dima. Dima tells Blackburn about what the Iranians are planning to do and how they stole nukes from Russia. Blackburn is just dead silent during that moment and doesn't question what Dima is saying, meaning Blackburn is just another playable character that does what he's told and always takes a leap of faith instead of preparing himself for various conflicts and scenarios in the game.
The worst example of my problem has to belong to "Homefront". In that game, you're playing as a helicopter pilot named Jacobs. In the game, just about every second asks for Jacobs to talk yet he doesn't. He doesn't talk smack to the North Koreans when they capture him. He doesn't tell his teammates (Connor and Rianna) to stop arguing when they do. He doesn't talk to any non-playable characters when you press the action button to talk to them. (All you get is them talking to you, while you're just dead silent.)
I'm sorry to have to say this about Call of Duty, but its latest games are actually getting it right (BLOPS1 onward) when it comes to input from playable characters, making them have an actual personality instead of just being brain-dead characters that are just there to advance the story.