Any time something is banned, it becomes more prevalent and governments lose any semblance of control that they may have had on a thing. Such a ban would also force people to re-think security. There is a false sense of security if a thing uses an encrypted transport, or has an encrypted disk. This only partially hinders accessing data by people that are not supposed to have it. I should not have to start documenting the ways to side-step current implementations of encryption, right? This is the Hacker News, so most of you should already know at least some of the methods.
OK, back to reality. A ban would legalize what is already being done. There would be no more need for individuals to risk their own safety by breaking gag orders, NSL's, court orders, et al. Everyone would be painfully aware of what is being monitored.
BTW, I am being partially sarcastic here. The 3 letter agencies are nodding their heads as they read this. A ban on encryption would be highly detrimental to their operations. Such bans would be dead in the water or have their teeth removed before seeing the light of day.