As an Immigrant...
As a non-EU immigrant to these isles, I can see the advantage of these cards, as long as the proper controls are in place. As other posters have pointed out though, the UK government is rubbish at putting in and maintaining said controls.
Although I'm a pretty staunch libertarian (Go Ron Paul!), and I think I'm exempt as I've already got my biometric ILR in my passport, I wouldn't mind having instant verification of my status. I'm still not a citizen so I can't expect the same treatment as a UK citizen in all situations. If it's only one hoop, then my employer can see only what they need to see (I can legally work) or the benefit office can verify that I'm entitled to a specific benefit, then I'm all for it. I don't want some busy-body at the council who doesn't like the fact that I left my bin out too early having the right to check my papers. Must be strictly need to know.
The real scandal is the current government's attempts to prevent families making a home in the UK, in practice preventing those on lower incomes from marrying whom they choose. The US government does this and it is disgusting. But that's neither here nor there I suppose.