Which stdlib functions and types should be marked as `infix`?

[OndrejSpanel]

Mentioned in a different topic, I do not want to hijack it, therefore I am posting the response here.

@odersky

For intersect and symmetric operations like it, there’s always the possibility to write them infix in backquotes.

I yet fail to see how is a `intersect` b better than a intersect b and why should anyone prefer the first form.