The idea is balance.

Yes, balance.

I used the C+D for an entire day today trying to see if I could not suck at it / if it was useful, and I've come to the conclusion that it's really for recon/sabotage spies and not for people who want to get lots of points / backstabs. It can let you be a huge help to your team by allowing you to find the opportune moment to sap a sentry or get rid of a teleporter so you can push forward, but unless you explicitly TRY to use it to stay in one area and backstab people you won't get many points. And if you're doing that you might as well use the regular cloak or Dead Ringer since you'll get points even faster.

I've found that it's more useful if you coordinate w/ your teammates / other spies ... I camped out behind some engies and waited for another spy to sap the teleporter entrance at the other team's spawn; after a while, the engy left to try to rebuild it and I uncloaked and sapped the stuff.