What advantage do I see for students in using flexible grouping? Well, I love that fact that it IS flexible. I can create heterogeneous groups for one task, and and homogenous groups for others. More importantly, a student is not delegated to a group forever. A student who learns faster doesn't have to wait for the group to catch up. A student who usually is at the head of the class but has difficulty with a particular concept can have more time.
Also, flexible grouping builds self - esteem. Since groups change frequently, no one can say"I'm in the dumb group." This was especially apparent when I had that tiny class. I did leveled texts in ability groups, but the selection in gender groups, which meant that Legend (limited), Chris (proficient) and Steven (accelerated) read together. Legend realized that he could read! Chris realized that he was a good reader.
Finally, I like being able to switch up the groups based on who needs a skill. Sometimes students have a good DIBeL and a good OAA, but still has difficulty with sequence, or Fact/opinion or (insert skill here). Building better groups means building better readers and mathematicians.