You'd probably have to go a long way to find a designer looking to develop a AAA MMO for a wide audience that wouldn't consider putting some sort of direct PvP (i.e. player vs player) combat in their game. PvP has come to be a core system in the MMO genre, up there with character creation systems or chat systems - a MMO would appear incomplete without it. It's also a system that attracts a dedicated audience.
This is why, intentionally or not, MMOs are starting to rely on baiting and switching when it comes to PvP.
Here's how it goes:
You devote separate media releases / interviews to PvP issues. Media articles talk about how important PvP is to the game (which it might be and probably is when players get to the 'endgame') and how many options players will have. A few write-ups of the PvP system sound awesome - you can take over buildings! You can enslave kingdoms! You can carve your name on the face of the world!
All this information drags the MMOPvP player into a frenzy. Finally, THIS will be the MMO that gets PvP right. THIS will be the game that satisfies my PvP desires! So the PvP guilds / sub-groups get together and plan to play at launch. This is because this will probably be the easiest time to make their mark - everyone is a newb and all lands remain to be conquered (or in-game equivalent). I have no stats regarding this, but I'm sure PvP commentary helps drive MMO pre-orders upwards among the PvP groups, who in turn try to convince others to sign-on.
Those PvPers in beta might be able to shed some light on what the PvP is really like, but often such information is fractured - some like it, some hate it. Beta is also not a good testing ground for PvP, since there usually isn't the critical mass of players to really test things out and combat functions change regularly, so that what works in one build doesn't work in the next.
The MMO launches and the PvPers arrive to find... their beloved PvP is not what was promised. Sure, you might be able to PvP anywhere in the world, but in reality only about 30% of it is open to such behaviour due to player numbers or player distribution or in-game systems designed to protect non-PvP players. In order to play the PvP, lots of time is required in PvE to get levels and / or loot. At launch, often key PvP systems are still out of wack, untested because not enough people had the opportunity to play test them thoroughly.
The great PvP swindle is a bait and switch to draw players towards an unreleased title. For all the alleged importance that some just released (AoC) and to be released (WAR) titles appear to put on PvP, the reality of it is that PvE still makes up the preferred advancement mechanism and the majority of the game. PvP works at the 'endgame' when enough people get there, but it rarely works prior to that point outside of the odd duel or skirmish. It is also a system that takes time to balance out - early PvP often sees in-game balance issues and / or bugs used to gain advantage that require quashing for things to become fair (or less unfair as the case may be). PvP only comes into its own after the MMO has been released for a while.
I'm not a PvPer. Nor am I here to complain that AoC didn't live up to expectations in the PvP side of things. But I've seen this tactic apparently taking greater hold - to focus on the PvP side of the game and ignore the PvE aspects during pre-launch media releases / previews - and find it a bit misleading.