为什么不总是在vue.js for loop中始终使用索引作为键？

console.clear()

Vue.component("item", {
  props: ["value"],
  data() {
    return {
      internalValue: this.value
    }
  },
  template: `<li>Internal: {{internalValue}} Prop: {{value}}</li>`
})


new Vue({
  el: "#app",
  data: {
    items: [1, 2, 3, 4, 5]
  },
  methods: {
    addValue() {
      this.items.splice(this.items.length / 2, 0, this.items.length + 1)
    }
  }
})

<script src="https://unpkg.com/vue@2.2.6/dist/vue.js"></script>
<div id="app">
  {{items}}
  <ul>
    <item v-for="i in items" :value="i" :key="i"></item>
  </ul>
  <button @click="addValue">AddValue</button>
  <ul>
    <item v-for="(i, index) in items" :value="i" :key="index"></item>
  </ul>
</div>