The Boston Marriott provides us with a wonderful example of why punctuation matters in this sign. Why does the lack of an apostrophe matter? Well, not only does it make the hotel look, well, dumb to guests, in this day and age, it also makes it look dumb all over the Internet. I saw it first on Seth Godin's blog, and have since seen it in about 15 other sites.
This is why every bit of printed matter, however humble, should have a proofreader. We can never say this enough. (I only wish I had a proofreader for this blog—I'm sure I make tons of mistakes even more egregious than making up a new word for more than one person of the female persuasion).