This is a very slippery slope.
People who leave the the country for "nefarious purposes" are not going to self-report their movements. A committed terrorist is simply going to bribe their way around visa stamps, or tear out relevant pages, or simply "lose" their passport proir to return to the USA (just stopping at the local embassy to get a temporary replacement passport for $55). Then they'll lie about their travels.
So in practice this "rule" will not help law enforcement one iota.
For example, travel throughout the EU (where a huge number of terrorist cells are located) requires no visa whatsoever, and there is nothing placed in your passport to indicate your travels within EU countries.You can run from Spain to Sweden without a single stamp.
So again, terrorists and criminals will simply lie about their travels. Only the sheeple will be forced to self-report to the USG regarding their private and personal activities.
This is a huge warning sign that we are heading the way of Soviet-style travel restrictions.
As I said, I am not nuts about this, but I want someone to come up with an alternative proposal.
It seems to me that there are a lot of people who gripe about situations such as this, but at the same time, should a US citizen travel to a foreign country and bring back sarin or smallpox virus, the very same people would be griping that the terrorists weren't monitored.
I repeat: HOW are you going to attempt to track the movements of suspicious US citizens?
Bingo. This is to ask why you are going to, say, Luxembourg, and are you taking any money with you? This will have no positive effect on security from terorists.