You can but it's inadvisable. There are two dangers with rhetorical questions:

1) The reader won't respond the way you want them to. You have very little space in an AWA, it's not smart to ask a question when you don't have a lot of time to convince them of your position. You could ask, "Did the author consider all the possibilites?" and they could think to themselves "Yes. They did." You have no way of controlling for that.

2) It's seen as "weak." A MUCH stronger statement would be "The author did not consider all the possibilities - he missed X, Y, and Z." You may look as though you yourself are unsure by asking a question. It's better (and more convincing) to make a strong, confident statement supported by evidence than a rhetorical question that you have no control over.

thanks myohmy ..that clears it up

I agree with what myohmy has said, and will only add that I have scanned the Score 6 sample essays from my OG and they did not ask rhetorical questions- though many have scored 6s and not followed the style/format of the samples.