I don't agree with ability tables because I feel it limits opportunity for all of the students, both GD and WT.
As I said my GD kid is often sat with ND children that need help to concentrate. From that she gets opportunities to articulate and breakdown what she is doing in order to explain them to someone else - GD kids often lose marks on tests because they don't bother writing the steps they took to reach an answer down. She is learning the point of doing that. She also gets a bit of perspective, empathy, fun and time spent with other people who think differently and have other ideas. Neither of these things take up that much time - but it's not a good thing for GD kids to just crack on silently, they need that time talking about ideas just as much as anyone else, so they are a real positive.
It's generally useful for the ND kids because she keeps them on track... Now, in fairness, my kid has ND siblings so they are used to viewing people with SEN and their contribution positively. The teachers are also quite considered about who they pair up and where they direct their or the TAs attention. I have never once had my child come home and complain about who they are sat with.
In terms of her ability to stretch, she is in mixed year classes so everything is always set up on a 'at your own pace' basis, and she has constant access to additional materials and challenges once the set work is done. Because this set up is for everyone, the children who are not working at GD also have access to these materials and if they hit a topic they get on well with also have the opportunity to try something more challenging, regardless of what level they are deemed to be at.
I don't believe it's a good thing for only certain tables to have access to challenging materials either. It's very limiting towards most of the class, who might be perfectly capable of doing more if given the opportunity.