A known bug [1] of Internet Explorer 10 and earlier makes the generic "serif" rendered poorly in Japanese, mixing one font (a Mincho font, close to serif) and another (said to be the Korean "Batang" font, close to sans-serif), character by character in one line. (screen shot in [2]) Because Latin-script fonts specified for headings in the current Vector skin are not applicable to Japanese scripts, the generic "serif" has to be applied. This results in IE users in Japanese experiencing a readability issue due to the forementioned rendering bug of the browser.
Note that this is not merely an aesthetic preference, but a problem close to breakage in the interface; when users see this anomaly in the font style, most would go to check and see if something is broken on their computers.
[1] http://answers.microsoft.com/ja-jp/ie/forum/all/css%E3%81%AEfont/76ecfcff-ca10-41c7-9a77-baa823ec89a0
[2] https://sites.google.com/site/jawpspace/jawp_font_ie9s.png
Also discussed in https://www.mediawiki.org/wiki/Talk:Typography_refresh#About_troubles_include_IE.27s_rendering_error_in_Japanese_Wikipedia