Of course an ESR (Enhanced Score Report) is not the same as an actual answer key. However, for high-level GMAT test-takers in particular, ESRs do help determine which answers they answered correctly (and by extension, the correct answers to those questions). That's why I would consider an ESR to be
similar ("akin to") to an answer key, in that it either confirms or disconfirms one's originally chosen answers as correct.
Let's say, for example, that I take the GMAT online one week, and you take it the next. I order an ESR (hypothetically of course), which confirms in about 48 hours that I scored a Q51 with every counted question correct.
Wouldn't that mean I would know the correct answers to all 28 counted Quant questions I saw that day, and that I could potentially share that information with you and/or other test takers, in case they saw some of the same questions? With an ESR in hand, I would be able to know—with 100% confidence—the correct answers to those 28 questions in advance (as best as I could remember them, assuming that my questions and answers weren't being surreptitiously recorded).
I wouldn't do that, of course, but it would be theoretically possible, and I suspect that GMAC fears the same, which could be another reason for the 2-test limit for the online exam.
If you say you're not "pro GMAC," then I certainly believe you...I mean, who is, other than GMAC and its employees?