Eric Nofsinger explained to 1UP that the main reason it was dropped is because it didnt improve IR tracking, and ultimately added nothing to the experience. High Voltage hoped that Wii MotionPlus would enhance IR tracking, in addition to enhancing the Wii Remotes motion-sensing abilities. Since Wii MotionPlus logically offers possibilities with melee weapons, they actually prototyped a few melee weapons that would take advantage of the peripheral. However, due to the games June release date there wasnt time to fully incorporate them into the final product. The developers ultimately scrapped Wii MotionPlus support and these specific melee weapons.
Nofsinger confirmed that WiiSpeak support is still included, and described to 1UP that it took a lot of effort for High Voltages audio engineers to do so. On the bright side, Nofsinger confirmed that WiiSpeak is a fantastic fit for the game, and the efforts have paid off since it is an excellent feature for online multiplayer.
I was surprised that they thought MotionPlus enhances both the IR pointer and motion sensing.