The most important statistic you can provide for this kind of argument is neither of the one's supplied by zMario. While New Zealand, West Indies and Pakistan may be weaker (which is a moot point to start with) than the other five Test playing nations, they are still deserving of their Test status whilst Bangladesh and Zimbabwe aren't. So what is the point in excluding runs scored against them?
http://stats.cricinfo.com/statsguru...3;spanval1=span;template=results;type=batting
Minimum of 20 innings (and has to have played in the last two years)
Michael Hussey - 67.19
Ricky Ponting - 67.10
Mohammad Yousuf - 62.21
Jacques Kallis - 61.08
Brian Lara - 59.49
Younis Khan - 57.89
Virender Sehwag - 56.98
Shivnarine Chanderpaul - 55.57
Mahela Jayawardene - 53.72
Rahul Dravid - 52.65
Kumar Sangakarra - 52.50
Kevin Pietersen - 50.51
But again, what does this prove? Absolutely nothing. There are so many variables in cricket that if you really do feel the need to use statistics as your whole argument you can't base it on overall average, because that shows nothing. For example, what does Sangakarra average when he's playing purely as a batsman? What does Rahul Dravid average when he isn't being forced to open? Michael Hussey has played only a handful of Test matches outside of Australia.
Far too many variables to rely on one number to try and make an argument.