Here's the thing... that never works. If you have two completely separate lives it will, but when you have your aides, who work on foundation, election and state department stuff (maybe not "work" but comment on, such as "I don't think you should take that $500,000 from that prince" kind of thing), you can never keep the two from co-mingling.
Keeping separate emails is basic practice for those handling classified information, and is a requirement of the position. This is explained to anyone who is given clearance/access to such information.
I don't buy the "she did it to hide illegal activity" because doing this was in fact a crime, which she was aware of--as all employees with access to this information have to take oaths, sign contracts, etc regarding what will be done / cannot be done. By choosing to break the law in a way that could not be hidden, she created a situation where she would be investigated at some point.
The fear wasn't of illegal activity being discovered, or at least it seems unlikely. When we take into consideration the sleazy and dishonest behavior of the person we are discussing however, what we could easily expect is that she didn't want things that may hurt her public appearance coming to light until after she was out of politics.
This is in alignment with the demonstrated aversion of HRC and her subordinates to FOIA requests. It seems more likely that she didn't want the public to see things done as SoS, at least not until later.
It just doesn't seem to hold that "the most qualified person to be President since FDR" was breaking the law in a very noticeable way, to avoid being caught breaking the law in a very not-noticeable way.
If she was doing this, it would actually support the "she's retarded" argument lol.
Of course, I really don't know why she did it. Her motives are conjecture. And in any discussion of her motives, I have to make that clear.