A practical aspect: I assume returning a named tuple is going to be very common. Within the implementation one might not want to deal with the element names.
def m(xs: List[Int]): (sum: Int, log: String) = {
val r = xs.foldLeft((0, "")) {
case ((acc, d), x) => (acc + x, s"$d$x")
}
// some work with `r`
r
}
unnamed <: named allows the above without the runtime cost of a conversion.