Extras should be able to respond appropriately to attempts to communicate. They should also exist unto themselves and add atmosphere to the scene appropriate to the world, or effect desired.
Basically, by having extras in your game you are trying to generate a particular effect. You are trying to convince the player that they are in a town, or city, or on a bus, or somewhere other than in front of their computer. The extras generate background information that is not plot critical, but necessary for the suspension of disbelief and the desired atmosphere. When encountered in non-interactive media these encounters are limited because they are not plot critical. Any non-critical information is typically removed for reasons of pacing and focus. Putting all sorts of other conversations into a movie would make the movie hours longer, and detract from the story trying to be told. With games, the same thing happens, but because the focus is more on player encountered story, rather than directorially controlled story, the pacing tends to be slower and amble more.
In daily life, there are hundreds of extras, none of which we typically talk to unless it is directed. But we could, and we would get a response. Typically this response would be a strange look, and them hurrying on their way. Occasionally, you might be able to get information, or elicit comments from strangers. More often, the reward for random communication is too low to encourage us to continue or repeat the process. Not so in games. Because the cast of extras often numbers in the dozens, instead of the hundreds, it is much easier to talk to everyone. Extras also tend to mill about purposeless. People don’t. People have things to do and places to be and getting in their way is usually a bad thing to do.
I would argue that the use of extras, and their level of responsiveness, should be dictated by the story being told. In most cases, I agree with you: extras should not talk. But they shouldn’t talk because the player shouldn’t be trying to initiate contact, not because they can’t due to lack of scripting or inability. Extras should be impossible to corner, and when they are should quickly extricate themselves (and even leave), keeping to the context of the game. Extras should be in inaccessible areas. Or extras should just be omitted whenever possible. If you can create a scene with the same level of atmosphere and feeling, without using extras, then you should. Because using extras poorly will break the believability of the world.
Alternatively, conversation with companions should be in place to detract from the desire to communicate with random extras. When you travel around with a group of people chances are you talk with them, possibly on a constant basis. Why would you strike up conversations with strangers when you have people you know, and have relationships with, close at hand to converse with? It is not the extra’s fault that the world and social interactions of the game lack definition. I would posit that talking with extras is often a player’s attempt to elicit natural communication from an incomplete world.