The recent calls by both the UK and US governments to force all to use an encryption that governments can penetrate at will is a very dangerous idea.
There are two main arguments against this:
1) Moral: The fifth amendment says "nor shall be compelled in any criminal case to be a witness against himself". Legislating encryption that can be broken forces us all to bear witness against ourselves by having a computer or mobile phone.
2) Practical: Do you trust the government to create a new encryption that only they can break?
If we're to do business on the internet (banking, speaking with lawyers, accountants, etc) we must always use the best security available (which is encryption).
If the government mandates weak encryption, criminals will be able to read our traffic too.