The attribute `rbspan` for ruby text is quite important for some use cases, i.e., to write a reference-like ruby syntax for coding readability and friendlier supports for ruby-less or ruby-disabled web browsers. (I've discussed this issue in public-i18n-cjk mailing list[1].)

[1]: http://lists.w3.org/Archives/Public/public-i18n-cjk/2013OctDec/0052.html

<p><ruby>
    <rb>明朝<rb>是<rb>中國<rb>歷史<rb>上<rb>最後<rb>一個<rb>由<rb>漢族<rb>建立<rb>的<rb>中原王朝</rb>，<rb>歷經</rb>12<rb>世</rb>、16<rb>位<rb>皇帝。明朝初期定都於應天府，1421年明成祖遷都至順天府。1368年，朱元璋在統一農民起義軍後，在應天府登基，國號大明。明朝初年，國力迅速恢復，經過明太祖朱元璋的洪武之治，勵精圖治並逐步恢復國力。

    <rtc lang=“zh-cmn-Latn">
        <rp lang=“zh-cmn”>（<strong>上方段落的普通話漢語拼音：</strong></rp>
        <rt>mingchao
        <rt>shi
        <rt>zhongguo
        <rt>lishi
        <rt>shang
        <rt>zuihou
        <rt>yige
        <rt>you
        <rt>hanzu
        <rt>jianli
        <rt>de
        <rt>zhongyuanwangchao<rp>, </rp>
        <rt>lijing
        <rp>12</rp>
        <rt>shi
        ……
        <rp>）</rp>
    </rtc>

    <rtc lang=“nan-Latn”>
        <rp lang=“zh-cmn">（<strong>上方段落的閩南語羅馬拼音：</strong></rp>
        <rt>bin-tiau
        <rt>si
        ……
        <rp>）</rp>
    </rtc>
</ruby></p>

From the code example above, we can tell that reference-like ruby is way easier to read than `<rb><rt>`-style syntax within a fairly long paragraph. We can make the best use of it with `rbspan` attribute, especially for complex ruby.

Also, for browsers that supports no ruby, or those users who disabled the feature, we can simply display annotations paragraphically (with proper punctuation and explanation in `<rp>`s) instead of following each character or phrase to distract.

Further more, if we wrap each word/phrase within each ruby element, it would be impossible to add cross-phrase elements round them.

From the code block below, if the author plans to add a hyperlink to the text ‘有聽著’ in the sentence ‘`你~`敢有~`聽著~`咱~`的~`歌~’, since the text are in different ruby elements respectively, they'll be forced to separate one hyperlink into two.

<ruby>你</ruby>
<ruby>敢<a href="#yes-i-do”>有</a></ruby>
<a href=“#yes-i-do”><ruby>聽著</ruby></a>
<ruby>咱</ruby>
<ruby>的</ruby>
<ruby>歌</ruby>？

Improvements,

<ruby>
    <rb>你
    <rb>敢
    <a href="#yes-i-do”>
        <rb>有
        <rb>聽<rb>著
    </a>
    <rb>咱
    <rb>的
    <rb>歌</rb>？

    <rtc>……</rtc>
    <rtc>……</rtc>
</ruby>

One hyperlink provides better semantic structure; while two break the simplicity of the syntax. The behaviour of the links would be a bit nonsense as well (such as hover, active and focus events, etc).

These could only be realistic with the existence of `rbspan`.