FullMetal's right. If you were standing on a manhole cover and said "
this manhole cover", it'd be pretty clear which manhole cover you're referring to (the one you're standing on). On the other hand, if you said "the
next manhole cover," it would be similarly clear that you're referring to a different cover.
Analogies are fun.
And speaking of analogies: colloquialisms. There was a time when I would have agreed with you, Geb, before I started to seriously study languages. Not so much anymore, though.
The thing with languages is, they're basically codes. Symbols composed of noises and/or text which represent individual concepts. A code only works if everyone involved knows the code. Colloquialisms happen when people decide to change the code, for whatever reason (usually because it's easier or they haven't been properly educated). The problem with that is that colloqialisms are by definition localised; not everybody is in on this new code, and that impairs communication, which of course is the entire purpose of communication.
Now of course I realise that this is the natural path of languages -- to build up colloquialisms and dialectical differences until two groups are no longer able to understand one another -- but that doesn't mean that we should allow it to happen without attempting to stop or slow it. Information theory dictates that any highly ordered system will inevitably decay into a less ordered system, unless acted upon by an intelligent source. We are that intelligent source. If we don't act to preserve the universal integrity of our language, it will cease to perform its primary function -- communication.
...you thought I was kidding when I said I was passionate about linguistics, didn't you?