I don't know if anyone has ever proved that consistency has value. But I have compiled two lists which you can see here. I took all the players who had 10+ seasons with 400+ PAs through 2005 (there were 504 players). Then I found the standard deviation of their offensive winning percentage (a Bill James stat that says what a team's winning percentage would be if all 9 batters were identical and you gave up an average number of runs). Since I used data from the Lee Sinins complete baseball encyclopedia, OWP is also park adjusted. Then that SD is divided by the mean OWP. This is necessary because players with high OWPs will see bigger absolute year-to-year fluctuations.
But then I wondered if players with extra long careers would be penalized. The reason is that when you get older, your performance can tail off very quickly and those very low OWPs increase your SD. So you get penalized for longevity. Then also, your career OWP falls and your SD gets divided by a smaller number, raising my measure of consistency (the lower the SD/mean, the more consistent). So I created one more list where SD/mean was then divided by the number of years. For Hank Aaron, he jumped from 112th to 12th.
On the first list (SD/mean), Dom DiMaggio is first. He only had 10 400+ PAs seasons. Once I did the 2nd list, (SD/mean)/Years, Mel Ott jumped to first. Dom DiMaggio dropped to 8th. Guys that get hurt by the 2nd list are the guys who lost years to military service in WW II. They get divided by a smaller number.