There was a time when the main responsibility of the wicketkeeper was to keep wicket, and occasionally contribute with the bat, but that was more than a couple of decades ago. The days of light hitting, defensive specialist glovemen such as Wasim Bari, Bob Taylor and Godfrey Evans are a thing of the past.
The graphic below (which can be enlarged by clicking on it) lists decade-wise averages of wicketkeepers since 1950, and it's clear that since 2000 the numbers have changed dramatically: from an average that used to hover in the mid-20s, it has shot up to more than 32 over the last decade.
Compared to the 1950s, the batting average of wicketkeepers has jumped up a whopping 63%. During the same period, the averages for openers went up by about 10.5% (33.42 to 36.90), for all top-order batsmen by about 18% (32.42 to 38.34) and for all tailenders by about 11% (14.05 to 15.63).
In each of those cases, the increase in averages is less than 20%, while for wicketkeepers the rise is more than 60% - that expresses the dramatic change in their role pretty vividly. In the 1950s, there were ten centuries for wicketkeepers in 505 innings - an average of one every 50.5 innings; since 2000, it has gone up to 107 in 2326 innings - that's one every 21.7 innings.
Over the last 20 years, 18 wicketkeepers have averaged 30+ and 8 of those have an average better than 40. Compared to the 20 years prior to that, Andy Flower was the only wicketkeeper with an average greater than 40, and only 4 others had an average above 30.