What happens when you send a TXT? You tap it in, press the button, and your friend on the other side of town gets your pointless, illiterate message. What people don't know is that the phone company takes a copy of every message and stores it for no apparent reason (actually, I can think of several, I'm just not sure which applies). And if the police take an interest in you, they can get a search warrant and read every message you've sent, without any of the checks and balances or protections that would be applied if they asked for a proper interception warrant.
At least, that's how it worked up until recently; now Vodafone have switched to a new technology which simply routes messages from A to B without storing them, and the police are screaming about it. Rather than do real police work, or obtain interception warrants allowing them to grab text messages in real time, they want the government to change the law to force telecommunications providers to copy every message, just in case they need it.
Think about that for a moment: our police want to archive everything we say, just on the oft-chance that one day they might be interested in it. But we don't let them force NZPost to photocopy every letter which goes through the mail, and we don't let them force Telecom to secretly record every phone conversation we have. We recognise and reject these as Orwellian demands, grossly invasive of privacy, and hugely open to abuse. Their demand that all TXTs be recorded and archived should be treated the same way.
Update: Stephen Judd makes a similar point here.
