Not quite true. Stewart played a lot both as a specialist batsman and as a wicket-keeper during the 90's - though it is true he played far more as a keeper after Russell retired he still had his fair share of games as a keeper during his early career. Between 1990 & 2000 he played 93 matches - 42 as wk, 51 as a specialist batsman (he played a further 42 tests from 2000 onwards, all as wk)
*. He averaged 34.0 in matches he played as wk. He averaged 46.7 in matches he played as a specialist batsman. Pretty compelling evidence that being wk can effect your batting!I'm not talking about ODI batting at all - it is pretty obvious I'm only referring to test cricket. Sangakkara averages 40.5 in 48 tests as wk. He averages and amazing 68.9 in 63 tests as a specialist batsman!McCullum's test batting has not got any better as he has become more experienced. He started off strongly - he was averaging a touch under 40 at test level after 10 tests. That dropped to 36 after 20 test, and down to 30 after 30 tests. NZ didn't find a better wk - McCullum gave up wicket-keeping in tests as his body wasn't able to handle the rigors of test match wicket-keeping. He is still clearly the best pure wicket-keeper in the country. He averages 43 as a specialist batsman, but only 35 as a wk (admittedly not a massive difference compared to Stewart/Sangakkara).Why didn't I mention Dhoni, Boucher, Russell, Healy, Marsh, or Dujon? Why
would I mention them? Barring Dujon (in 2 tests) none of them have played tests as specialist batsmen, so you can't analyse how wicket-keeping effected their test batting! Flower is one I had overlooked, but he has only played a small percentage (8/63) of his games as a specialist batsman. I'm not sure you are following what I am saying - I'm not saying you can't be a good test batsman if you are a wk, just that many players struggle to average as much as wk as they do as specialist batsmen at test level.
*All stats courtesy of the statsguru search engine on cricinfo.