{For those of you abroad who have one, are there any obvious advantages or issues?}
As a credit size card it can be carried much more easily than a passport so not an 'imposition' in a physical sense.
Deliveries (UPS etc) often require ID before handing stuff over.
If you happened to be stopped by police, you are able to determine who you are 'instantly' and as I suspect most 'incidents' where people might be checked, they have a pretty good idea who they are looking for, you can go on your way within minutes.
For people who still use cheques (still common in France) they are used for ID by supermarkets.
The only reservation I would have is to actually carry one ALL the time.
As a thought, when the IRA 'remodelled' Manchester Arndale centre, how much did that cost and if having ID would provide a reasonable approach to reducing the risk of terrorism and fraud it would seem reasonable to me.
Obviously no system is perfect as even if half the UK were 'security forces', statistically a certain percentage of those will be 'bad apples'.
As this train of thought started with voting, it is not beyond the wit of 'man' to use domestic bills or whatever to prove ID at a polling station. It would take a concerted effort to get a significant number to really 'swing' a vote and as DGR would probably point out, even if your candidate of choice gets in, they are still unlikely to actually carry out what they promised during campaigning.
There will always be cheats in the system.