Jeffrey Goldberg at Agilebits, who make 1Password, has a great primer on why law enforcement back doors are bad for security architecture. The entire article is worth a read, presents a solid yet easily understood technical discussion — but I think it really can be distilled down to this:
Just because something would be useful for law enforcement doesn’t mean that they should have it. There is no doubt that law enforcement would be able to catch more criminals if they weren’t bound by various rules. If they could search any place or anybody any time they wished (instead of being bound by various rules about when they can), they would clearly be able to solve and prevent more crimes. That is just one of many examples of where we deny to law enforcement tools that would obviously be useful to them.
Quite simply, non-tyrannical societies don’t give every power to law enforcement that law enforcement would find useful. Instead we make choices based on a whole complex array of factors. Obviously the value of some power is one factor that plays a role in such a decision, and so it is important to hear from law enforcement about what they would find useful. But that isn’t where the conversation ends, it is where it begins.
Whenever that conversation does takes place, it is essential that all the participants understand the nature of the technology: There are some things that we simply can’t do without deeply undermining the security of the systems that we all rely on to keep us safe.