http://tools.ietf.org/html/rfc5988#section-3

   In this specification, a link is a typed connection between two
   resources that are identified by Internationalised Resource
   Identifiers (IRIs) [RFC3987], and is comprised of:

   *  A context IRI,
   *  a link relation type (Section 4),
   *  a target IRI, and
   *  optionally, target attributes."

The Link header happens to use multiple space separated rels for
brevity but that actually represents multiple links that share the
same target IRI, each with a single rel.

From a design point of view, prematurely optimising away the common
case (single rel) in favour of multiple rel doesn't make a great deal
of sense when the former is supported naturally by JSON. I think most
API consumers would appreciate keeping this basic necessity (selecting
a link) as simple as possible.

At the end of the day, if you want "multiple rels" that are similar to
the Link header you can achieve the same effect simply by having
multiple links that all point at the same target, e.g. below I have
represented /bob's conflicting emotions toward /ted :

{
  _links: {
    self: { href: "/bob" },
    hate: { href: "/ted" },
    love: { href: "/ted" }
  },
  name: "bob",
  description: "a confused little man",
  age: 50