Well, this is a matter of taste. IMO, a templating language for
HTML/XHTML output *should* be XML-based, and mako is not
(http://www.makotemplates.org/).

Genshi supports XML-based templating
(http://genshi.edgewall.org/wiki/Documentation/0.5.x/templates.html#id5),
and it supports text templates as well, which could come in handy for a
configuration wizard (issue 2550515).  I can imagine that processing
instructions can be useful sometimes, e.g. to change a variable (append
to a list) in conditional code.

However, lots of Roundup folks like TAL/METAL, and we don't even make
use of it's strengths to the full extent.  If done properly, TAL
templates can be viewed with a browser and give a good impression of the
interpreted template.  I'm sure our templates can be optimized for
speed, too (e.g. multiple calls to context/is_view_ok could be avoided
by storing the result in a variable).  And we could update to METAL 1.1
(http://wiki.zope.org/ZPT/METALSpecification11), which would allow to
extend macros.

TAL/METAL is standardized; there are non-Python implementations.  I
don't consider some shorter attribute names ('py:if' instead of
'tal:condition') justify a complete switch of the templating language.

Thus, if you prefer Genshi, you are invited to make a suggestion for a
pluggable template engine interface.  Theoretically, the template
language used is specified in the template itself (for the XHTML flavour
of TAL, at least)... but perhaps we should stick with one of those
choices (TAL/METAL or Genshi) per tracker instance, or even per Roundup
installation.