ins button in text editor produce semantically different HTML then the U button at the visual editor

[mark-k]

U button at the visual editor makes something like

<span style="text-decoration: underline;">text</span>

while the ins at the text editor makes

<ins datetime="2013-01-30T05:44:49+00:00">text</ins>

It is actually not easy to understand that the intentions behind both are equivalent - underline the text*, but semantically they have different meaning, and in the context of WP usage the styling one has probably the more correct meaning.

Alternatively, if "ins" is important html tag then why it is not possible to add it in the visual editor.

related #20149

*Browsers by default underline "ins" but twentyeleven and twentytwelve remove it.