If I can take the last bit first mate, yes they do work in terms of restricting access to secure sites(and sometimes the rozzers will let you off for speeding if you flash one, allegedly ). I am not aware of an MOD 90 - mil' ID card - being cloned or forged but that's not to say it hasn't happened. The ones that have beem lost couldn't be used by anyone other than the card holder unless the bloke on the front gate was an utter mong. However all forces personnel agree to surrender a good chunk of their civil rights in order to serve so carrying an ID card isn't an issue. I don't think it's on to take the same line with the general population who have certainly not consented to giving up those rights.
On your benefits point an ID card could be used to verify who the claimant is but I don't think that is where the fraud is happening, it seems to be more about people claiming who aren't entitled to at all. Either way that wouldn't be solved by ID cards and immigrants who are illegally claiming benefits wouldn't have them anyway so it wouldn't work as a catch all solution imo.
Biometric passports I think we both agree on, unfortunate but probably unavoidable.
I'm not quite sure what you are getting at with the banks, they seem to manage perfectly well now. Unless the government gives them the extra option of demanding "achtung papers!" - which they would no doubt take up - I don't see why it's relevant. Are you proposing they could be used to prevent credit card fraud? *a subject I know naff all about btw*
As for going to the gym I wouldn't know about that anymore
We seem to be starting from the premise that 'we' want ID cards and we will find uses for and reasons why we should have them, reverse engineering the issue. It just seems a little perverse to me and from an economic POV vast expense for no appreciable gain.