Interesting thread this. A few points haven't been made yet.
It IS interesting that many newer cars have the indicators so close to (or even within) other lights that they're very hard to see. Especially when those other lights are on all the time in the daytime anyway.
There is a lot of genuine confusion about the correct way to signal on roundabouts, particularly mini-roundabouts. Lots of folk I've talked to will argue that they're turning right at the roundabout - how can it ever be correct to use the left indicator? In the face of that kind of mentality, it's almost useless to continue any discussion.
In many places, this persistent and widespread failure to use indicators at roundabouts has led to the roundabout effectively failing to function - traffic can never join the roundabout smoothly as no-one knows whether vehicles approaching from their right is going to turn off or not. To counter this, roundabouts are being re-engineered with kerbs or barriers separating the lanes into which traffic has to separate depending on where it's going. This means that you don't have to signal, or relyl on anyone else doing it.... but it's a very expensive way of tackling the problem.
Finally, on the point of signalling or not signalling when there is or isn't anyone who could benefit from the signal. When you see a motor vehicle signalling, yes, the only safe interpretation is that the indicator works. But if the first time you see a vehicle signalling, the indicator is already flashing, it's possible that this is because the driver has been negligently driving along with it on for some time. But if it only comes on when you've seen each other, it's reassuring.
The fresh signal, made in response to the recognition that another person needs the information, is so much more believable. It not only tells you a manouvre's likely to take place, but that the driver in question has probably seen you (or someone) and is paying attention to what they're doing. Okay, you'd still be wise to wait for other evidence that there's no likelihood of a collision before committing yourself...
I realise that in most urban situations, there'll almost always be someone around who could do with knowing what you're going to do, but that's not the case elsewhere.
Oh, and finally, sometimes when cycling, it might sometimes be wise deliberately NOT to signal - it's best to keep others guessing as this gives you space you mightn't otherwise get.