Getting a bit off subject I know.....
I think the problem with this approach is that it depends on knowing that there isn't someone who may benefit from a signal. I definitely believe that if you don't get into a systematic approach, then you're going to miss a time when there is someone who would benefit from a signal, but who you just haven't seen (given human nature, I'd say that's very likely). Additionally, I think the use of a signal (when used in a systematic approach) is more likely to prompt use of proper observation (e.g., if you get so ingrained in using a signal, you know you should look and observe before putting it on, so it becomes an automatic action). Also, and while I've never had to put this to the test, I'd think in the case of an accident (where perhaps you had no awareness of another road user) and if you are using a signal, it may help you in terms of insurance and clarify your actions if you were not at fault.
I agree that most times sub conscious positioning on the road let's people know where you are going, but that also presupposes that you aren't a terrible road user with no clue of road positioning, or decide at the last minute -but then I guess signalling might be the last thing you'd think of doing

. I also think nothing's perfect; one of the problems with using a systematic approach is that if you're not careful you can go onto auto-pilot and while you make the motions of looking into a mirror or looking behind you, you're really not observing -just following a series of motions. Course, that can be applied whether you use a signal or not -which is why I prefer the systematic approach of always using a signal. It just seems to me there are too many reasons to use a signal rather than use them when you think it will only benefit someone (I'm open to be persuaded if there are really good reasons!). Having said that, there may be times when it is beneficial to purposely not use a signal in terms of safety (e.g. pot holes in the road, iffy road surface -done that quite a few times when you need full control of your bike) or to deliberately obfuscate intent (maybe shielding someone on the road?) but I can't recall coming across that.
V for Vengedetta, my copy of roadcraft says that there is no reason to signal when there isn't someone who may benefit from the signal. There are mixed opinions as to how sensible this concept actually it. However, signalling becomes fairly irrelevant most of the time when a vehicle is moving as (sub)conscious road positioning tends to give a much clearer indication of what the driver is planning.