Her defense at this point is absolutely outlandish. "Convenience" is absolutely against govt IT regs, and quite possibly illegal. Didn't want to carry two phones? Suck it up buttercup! I had to carry three: My regular work phone, a 24/7 emergency phone for one of my duties, and my personal phone for personal stuff. It was made quite clear that there would be no number forwarding or anything else to reduce the number of phones I had.
I could have done limited personal stuff on my regular work phone, as gov rules allow cell phone users to make personal calls as long as they don't go over their contract ceiling on minutes or data, or do so during company time. I, and many others, chose not to do so, not just because we didn't want the gov snooping in our private stuff, which they could with a configured phone (all work phones had to be configured for access by IT), but gov smart phones are so locked down that they are practically dumb phones, so carrying a personal phone let you use full smart phone features for your own stuff.
I will say that this is not just Hillary. The private email server certainly is all her, but in my time, I saw a lot of people that were SES who totally shined on regs regarding phones, tablets, laptops, etc. because, same excuse as Hillary, the gov configured devices were "inconvenient", and they figured they were high enough up the ladder that they didn't have to answer to anyone, and mostly they were right, since the highest IT people are generally GS level.
Absolutely most gov IT devices are locked down and inconvenient to use, but tough tiddlywinks, that's the way it is, and if you want to change it, then use the power you hold in your high level position to affect change. Instead, I cringe thinking about how much sensitive information sits on personal devices with giant security holes. Not to mention the loss of access to it by the govt and taxpayers.