Joining Arabic Letters

I come back to
 http://www.unicode.org/mail-arch/unicode-ml/y2012-m03/thread.html#11

A similar problem of showing non-joining, isolated Arabic glyphs
can be seen in the attached file. Both Internet Explorer 8 and
MS Word 2010 display isolated glyphs in some cases.

I think a better idea is to have joining glyphs always even for
different typefaces. At least the Unicode Standard should say
what should happen when Arabic characters of different typefaces
follow each other.