The motivation is stuff like this:

zip(iota(10), iota(10, 20))
.map!(t => Tuple!(int, "a", int, "b)(t))
.map!(t => with(t) a + b);

which, short of having some proper tuple unpacking syntax (that would be so, so good), is nicer than the current

.map!((t) { with(t) return a + b; });

that might seem like a small change, but I'm having a hard time selling D and functional programming to scientists with all the heavyweight syntax, they are spoiled by the inefficient but neat-looking numpy model.