Cake players are a liability in T20.
You are better off having a slogger who can just dial his game down a notch if the other guy is going great guns at the other end. I remember Guptil just holding up an end for Baz to go full psycho on England during the ODI world cup and it didn't seem to matter that Martin took 2-3 balls to get a single instead of being able to score a single at will.
You could have argued that we should have had some single rotator to bat with Brendon during the world cup but we stuck with Martin and he came through when Brendon failed e.g. against the Windies.
My main argument is this tournament and this game:
Final: Australia v England at Bridgetown, May 16, 2010 | Cricket Scorecard | ESPN Cricinfo
Michael Clarke decided he would be a cake player/rotator throughout the 2010 T20 world cup. In the final he scored 27(27). This chewed up 27 valuable balls for a substandard return. David Hussey was worse 59(54). England chased it down with 2-3 overs to spare.
You need non stop sloggers from 1-6 and maybe a recovery engine room with an Andy Ellis like player at number 7 to limp the team home to a score like 130 if everyone has a bad day at the office.
The only other reason I prefer two sloggers together rather than a slogger and a rotator is from my own experience. Sometimes I am going great guns with my own batting. When I see you hit a four it relaxes me and takes a bit of pressure off.
Finally this is a throwaway point and may not apply to international cricket, if I am in the zone and killing the bowling and then suddenly you hit 2 or 3 fours in a row then they have to keep the field out against you as well which makes it easier for you to pick singles for me.