Taunt, while not a necessity, is complete cheese in VGC.
Taunt is basically the anti-support move of VGC. Sure, it doesn't have any entry hazards to worry about, but there are many, many more awful things to face. These include:
• Trick Room
• Dark Void Smeargle
• Follow Me / Rage Powder
• Substitute Heatran / Mega Metagross
• Thunder Wave / Will-o-Wisp
• Minimize Clefable (-_-)
Taunt is such a great tech it's not even considered tech anymore. Locking the opponent out of Protect, stopping SubTran and friends, being able to bypass redirection, avoiding TR and Thunder Wave, guarding your Kangaskhan from Burns, avoiding the oh-so-ragequit Dark Void and Spore, and slapping Minimize Clefable are just a few of the joys of Taunt. You can also rid the opposition of their Wide Guard or Quick Guard support, leaving them open to attack. Say good-bye to Swagger shenanigans. The list goes on.
TL;DR Taunt is so rediculously useful that it's almost a crime to consider not having. It's not necessary, but it's a really handy tool that can help in a lot of situations.
Common Taunters in VGC