It's definitely true that "you can't please everybody", but developers should be careful about repeating that saying to their potential audience. It's easy to misinterpret as a dismissal of people or a side-stepping of the issue at hand.
In the first case, potential customers might wrongly assume that they will not enjoy your game because they're not members of your primary audience. But it's common that, while primary features of a game don't appeal to a particular gamer, its secondary features are enough to satisfy that gamer. So you stand to gain by not being dismissive of gamers who seem to have found your forums completely by accident.
In the second case, you're preaching to the choir. Gamers already know you can't please everybody, and reminders generally aren't useful. Even under the most horrific deluge of forum rants, bringing up that saying isn't going to calm anyone down or bring perspective to the discussion.
In face-to-face conversations, a calm and genuinely-sympathetic tone helps to calm angry and frustrated people. Unfortunately, tone can't be communicated as effectively without body language, so calming people through writing is much more difficult. Emoticons (smileys) help. Jokes help, though too many can make it seem like you're not taking the issue seriously. Also helpful is using longer sentences. Short sentences often feel terse, like the writer is irritable or impatient. But genuine patience and sympathy goes farther than any rhetorical trick, because a writer's mood tends to bleed into the writing.
Just because something is true doesn't mean it's the right thing to say at any given moment. "You can't please everybody" is the sort of truth that should be kept under your hat whenever you're dealing with customers. It's a saying to repeat to your fellow developers, not to your potential audience.