Looking this closely into packets can raise privacy concerns: can DPI equipment peek inside all of these packets and assemble them into a legible record of your e-mails, web browsing, VoIP calls, and passwords? Well, yes, it can. In fact, that’s exactly what companies like Narus use the technology to do, and they make a living out of selling such gear to the Saudi Arabian government, among many others.

This wouldn’t be an issue except for:

The userbase for strong cryptography declines by half with every additional keystroke or mouseclick required to make it work.
–Carl Ellison