I very rarely use stop signals, they are sometimes helpful when you know that you will need to stop at an unexpected place or for a hazard only you might see. One place I use the signal is here:
http://g.co/maps/c28ke
I need to exit this busy road on to the provided cyclepath by going in between the two bollards on the left of the road, just behind where that red car is in the streetview.
The queue of traffic on the left (of which the streetview shows a bus, silver pickup, red car) is in a filter lane waiting to turn left just beyond the shrubbery.
This is quite a quick road in that I am regularly at 25-30mph going down it, with vehicles happily speeding at 40mph at times. I have to come down to about 10-12mph and turn quite sharply to have a chance of getting on to the lowered kerb without just falling over sideways so I use a "slowing" signal on approach. Even if the drivers behind don't have a clue what it means, they are at least more likely to sit up and pay attention.
Ultimately, if I can make drivers slightly confused and thus more cautious (without prompting them in to irrational behaviour of course), this is probably a good thing for my safety.