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.