If somebody hands you a Pale Ale, you have certain expectations about what you’re getting. If it’s the color of a porter, you’re going to have that “this isn’t what I ordered” feeling.
Likewise, if you’re in a bar where they have English Ales on tap, you have a reasonable expectation that the ESB is stronger than the special, which is stronger than the bitter.
Well and good. That’s the right use of styles. Words should mean things.
Some folk seem to take this proper need for defining a style a little too far. In my opinion, there’s usually not a sharp line between one style and the next. There’s some overlap where, for example, a porter could be very like a stout, or it’s a matter of a coin toss whether an ale is a brown ale or a mild.
That is, styles have fuzzy edges — a fact that is recognized in the BJCP style guidelines.
Many styles are quite broad and can encompass multiple stylistically accurate variants.
A few times I’ve run into people who are a little too particular about style definitions. It’s as if they feel the need to show off their beer knowledge and be persnickety about minor things. I find it tiresome.
I like beers to be faithful to their styles (or to come up with a new style, if that’s necessary), but I don’t like to be too precise about it.