This has been biting me a few times now and I have the experience to override browser views, but this is really beginner unfriendly that we can customise more and more since z3c.jbot 1.1, but not the main_template itself anymore.
From what I get from the discussion is that the problem is a not well understood edge case in Chameleon, difficult to reproduce isolated. It was not fixed, but a workaround was applied.
I can identify at least two things why we do not want this:
no easy customization
template is instantiated on each call 2 times (vs. 1 time at Zope startup only before), so a performance issue (probably a small one).