One strength of the MGMAT/ARCO tests is that they are truly "adaptive" unlike most of the tests given by the other test companies. In addition, the MGMAT/ARCO test have a very nice summary of all of the questions you have taken, their difficulties levels, etc. Very handy for keeping track of your progress. Finally, there are a lot of tests (even top scorers can take the test about 4 times before running out of the "very difficult" questions) as opppose to the ETS version which just has two.
I cannot speak for Zeke, but it is my feeling that he believes it is more important to mimic the adaptive experience as much as possible.
I do not have a problem with the ARCO tests -- i actually used them to prepare for my personal GMAT and did just fine.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993