It always amuses me that given this is supposed to be a forum board for mil / ex-mil that so many are opposed to carrying an ID.
Nothing to hide....nothing to fear
Which is fine in theory, but when (given the scheme that was planned for the UK) the particulars are put under closer scrutiny the theory becomes a little more questionable.
For starters this was a government run, IT-based scheme. I don't think it is outlandish to worry about the security and accuracy of such a system given the UK's track record with national government IT systems. Considering this would have been a system with which individuals could prove their legality for a number of reasons I'd say security and accuracy were major concerns.
Then its worth bearing in mind the sluggish nature by which those controlling such government systems adapt to threats from those who may be interested in accessing the information contained within, such as criminal enterprises. Given the breadth of the previously planned system there would have been a wide range of personal information available to those inclined and able to access it in a single one-stop shop.
As has already been stated though, the existence of ID cards is no guarantee of security, indeed an over-reliance on such a system when criminals and potential security threats may have the ability to produce fraudulent cards is actually a dangerous overall weakness. If you can get into the system (and any system is fallible) then a fake ID isn't actually fake at all when consideration is made to said system being the likely means of clarification.
Yes I have a 1250 (it'll be a cold day in hell before I call it an MOD90) but it has a photo of me on it, a few written details and little more else. The proposed UK ID card would/could have had a wide range of details from fingerprints, iris scans, biometric data and a host of other data potentially useful to nefarious characters and all, in case we have forgotten, controlled by the lowest bidder for a government contract.
I for one am glad the scheme died an early death.