Not signalling left then veering off without warning also tends to irritate other road users though. Presumably as a cyclist you expect motorists to signal their left turns? Well, they also expect us to signal the same! The Highway Code says something along the lines of "always give a signal where it would be useful to another road user". It applies to everybody.
The only situation where a left signal is appropriate is where the only other vehicle in the vicinity is one waiting to come out of the road you are turning into - a courtesy signal.