This argument gets thrown around a lot on these forums and here's why I disagree. 1. Horde/Zombie/Firefight modes should be an independent challenge. When gamers party up and launch a horde mode they should be acutely aware of the impending "gauntlet" awaiting them not just a couple minutes of twenty normal mobs haphazardly thrown at them in the middle of a story mission. 2. Horde modes provide an escalating challenge that can be "opted" out of through various checkpoints or milestone. This allows gamers to continue testing their skill, strategy and gear every rung of the ladder they are able to complete. 3. Through various goals/leaderboards horde mode is able to provide an incredibly competitive challenge to players while being a completely cooperative experience! 4. A never-ending horde mode provides infinite end-game content. Many of us have beaten VOG hard and Nightfall every week but in a horde mode I can always better my last high score. It is a constant challenge. With all of that out of the way, I can tell you my opinion is that all of the defense missions in the story were brought on by laziness, lack of time or inept design. The art for this game is amazing and level design offers hundreds of fantastic varied locales, which is why I think the mission designers just took what the art team had in place and threw together some simple routes the mission would take. When faced with enemy placement and guiding the minute-to-minute gameplay Bungie took the easy way out. They picked the "cool" areas that they wanted to show off and threw in a locked door and a couple spawn points or drop-ships and PRESTO, you have the formula for the entire campaign. :(

It's better with some kind of objectives added to it, like Space Marine's Exterminatus mode. THAT was fun.