EuroTrash

You're unlikely to ever be asked to show it when you're out and about Tweedie, except when you're entering France from abroad.
When you're most likely to need it is if you have to enter the number online for certain administrative démarches.

You're the opposite to me - I carry everything with me in my handbag, specifically so I know it isn't being robbed while I'm out of the house. I get regular panic attacks and I need to be able to look in the special pockets in my handbag there and then, so I can tick my essentials off as present and correct - passport, driving licence, purse with carte vitale, CdS and bank cards, phone, keys. (I'm still damaged from my Italy experience, which was 7 years ago now so it's looking like I'll never actually get over it.)

Known Aim

I am not sure if carrying a photocopy is a good idea.

The chip in the Smartcard has all your info - name, address, DOB, place of birth and biometric details. That immediately says you are who you claim to be.

If you carry credit / debit cards, this is far less burdensome.

dmu

This is what your average French person must do with their ID. You never know when you need to identify yourself. If you don't want to carry the original all the time, why not scan your CdS and carry it in your Smartphone, and if there's a contrôle where, heaven forbid, they need your biometric details, you must surely have time to present the original to the Commissariat or Gendarmerie. Does Mrs. TP carry the original of her ID?
I'm like ET in this respect, carrying all originals (she forgot to mention car papers!) in my handbag, with photocopies of everything kept at home, in case it's all lost or stolen out of doors and I've got all the references when applying for duplicatas. But I appreciate that men don't usually carry such large accessories....

Chatter Static

Ours turned up so it was two weeks from handing over fingerprints. On a side note can the titre de sejour be carried as Id instead of one's passport as it fits in a wallet?

EuroTrash

Well Orange's website accepted a scan of my (invalid!) CdS as ID to register a new SIM card, having rejected a scan of a perfectly valid passport because it insisted my passport number was wrong. Go figure that one!

Chatter Static

Well it's good to know, in all fairness you can't have a CDs without a passport so you'd like to think logic applies. But then again we are sailing on the seas of cheese.