I hate the Bungie/343 philosophy of only including things in the game that "make sense" in the storyline. Do they really think players give a shit about maintaining story coherence when it comes to multiplayer? I loved the spike grenades. They were a great addition to the series and had really awesome physics. It doesn't matter to me that the Brutes are no longer in the game. The Brute shot and the spike grenades and even the spikers to some extent were interesting weapons that should be included in Halo 4, continuity be damned. I also don't like how 343 went out of their way to fit the multiplayer into the fiction with their Infinity ship. In all my years of playing Halo games it has never occurred to me to ask why a bunch of red and blue spartans are fighting on a map. I didn't need it to "make sense." It was fun and that's all that mattered.
I think Guardian was the best Halo 3 map as well, but I loved Narrows and The Pit was a good map (not for its style but for its design). Last Resort, High Ground, Construct, Rat's Nest, and Valhalla were also good maps. The only ones that sucked were Isolation and Blackout (should have been a proper Lockout remake).
In Summary: Please include Spike Grenades and other Brute-associated weapons and vehicles because they were great additions to the series. Story continuity means nothing to multiplayer.