I find it annoying also, with one exception: it does help to explain typos when you see a sigline indicating that the message came from a very tiny keyboard.
And let's not forget about the AutoCorrect feature that can also be the cause of some pretty strange substitutions, as it tries to make sense of the mistakes caused by the tiny keyboards. Heck, there's an entire
site devoted to the hilarity caused by AutoCorrect.
I shut it off for myself by about the third day, when the novelty of it had worn off. I didn't want to give the free advertising, or to sound like I was gloating. However, that little tag line, especially when it mentions an iWhatever, goes a long way towards explaining away strange word choices, even if the individual words are spelled correctly. Still, I have opted to NOT include it as part of my signature.