Thing is from a teacher's point of view it probably makes the classes easier to handle. They mix up any children who misbehave together, and share out children who need extra help etc.
I'm sure they don't want to upset children and break up friends, but actually that's usually the parents that end up dealing with the fall out, so that goes down the priority list. That may be cynical, and I accept that it does take a lot of time figuring out the forms, but I think friendship comes at the bottom of the list.
From parents' point of view. What I notice is that there's two sets of parents that like the mixing of the class. One is those with confident children who make friends quickly. They just add the latest set of friends with easy to their mix. They tend to mix with those who are also socially confident, so the parents have the view that it's great for making lots of friends.
The other set are those whose dc have had issues in the class. Sometimes it's just another child they don't get on with, sometimes it's just no friends, sometimes it's the child's behaviour being the problem and the parents hope every year that they won't be the "naughty one" in this class.
Personally I think if it's good for the children to be mixed up every year and make new friends it equally applies to the teachers. I think any school that does this should link with 3-4 other local schools and the staff be mixed up every year.
Our head disagrees, but all his arguments for mixing the classes up would apply equally well to the staff. (good to work with different people, stops people relying on one person etc.)
At our school they also ask every child to write down 10 children from their own class they want to stay with. Think back to your class at school; would there really have been 10 children you wanted to stay with? Generally if they get child 1-2 on the list they'll be happy. If they get 9-10, well I know I wouldn't have been able to name 10 people I'd have wanted to be in the same class. As my dd1 put (they didn't mix back then) "there were about 5 people I'd have wanted to stay with, about 5 people I definitely didn't and 20 people I couldn't have minded either way".
What's more, even with 10 children on their list and the "guarantee they'll be with one" there are several children every year that I am aware of get none. And it's never the confident ones, who seem to manage to stay with the same children every year right the way up.
The other issue I have is if they are split from their friends is they have no way of requesting the next year that they go back together again. They can't name children from another class. And at 5 form entry, the chance is not great.
Op, don't feel bad about not going in. I doubt extremely it would have made any difference at all. one parent going in saying "My dc wants to be back with their friends" is not going to change their mind on not mixing. They might have told you sooner that they weren't, but I suspect you'd have got along the lines of "they play nicely with lots of people in their form, and they play with their friends at lunch so it doesn't matter".