As I posted above I did believe the theory that Clinton implemented this scheme so that she could keep using her Blackberry. It seems like the facts have gotten in the way of my theory and I was wrong.
Now I'm back to the theory that she did it to reduce the possibility that her SoS emails would be made available to the Republicans and to others via the FOIA. It seemed unlikely to me that she would have taken such large risks to accomplish this but that is currently the only viable theory that has been put forth and it now appears that Sketic Ginger is entertaining the idea that this is the case (although it seems she thinks it was OK since the Republicans are such evil crafty devils*).
It is possible to relentlessly put forth the idea that the only way you could know that is if you could read minds. I suppose so, but do you give the bank robber a similar out: It looks like he was there to steal money, he did steal money but what was his motivation? Maybe he was looking for adventure. Who can know what motivates people if mind reading isn't possible? As a practical matter it doesn't matter all that much. The law, reasonably enough, doesn't require absolute determination of motivation. It looks to me like the you-can't-read-minds group here has decided that in this special case it's the critical issue. If it isn't possible to have read Clinton's mind her motivation is unknown (and unknowable) so her email scheme was OK. Maybe it was all part of her plan to end world hunger, who knows why she did it? But even after all this time she hasn't put forth a plausible explanation as to why she did it so it's extremely likely that she did it for the obvious reason: to hide her email communications.
ETA: * I agree with SG on this to some degree, we part ways I think mostly in that I don't think it's a legal defense and I don't think it's an ethical defense of her actions although I sympathize with Clinton over the outrageous behavior of the Republicans with regard to the hyper politicized attacks on the Clintons.