You are right. I don't want to have to socialise just to add a bit of voice to my game characters.
If I have to, I'd rather ship without voicing any of them.

The system you described would mean only the biggest names get paid

Rather, it's more like, we as the user get a greater variety of background NPC banter, for the same game price.

Take X4 for instance. The only banter we get is different types of "hello".
Only in cases of quests, is there any dialogue variety. When there is any such banter out of quests, it's mostly incoherent (or was that another game, I need to check again).
It doesn't really make sense that 2 or more people meet in a docking area, say, "Hi", "Hello", "Good day to you" and then just keep on standing staring at each other's faces as if they were using some sort of telepathy, or just staring at each other without any conversation.
It would be fun to be able to have conversations that, while clear that they would not be able to yield any Quest, should still have variety enough to be fun when the player stops by, eavesdropping.
This sort of thing is there in a lot of games by high budget studios, while at the same time, the games have pretty large file sizes.
This way, we can reduce both production and distribution costs.

And the VAs, they don't need to do all the work of speaking each dialogue every time the story writers come up with new banter, but the studio will be getting their voice for those lines, essentially increasing the value of the licensed TTS package, meaning the VA gets more work done than the work they do and gets paid more (well, the last part depends more upon the market condition).