You could send Warlocks anywhere you want, they just would not get a boost for the mission. Like sending a Warlock to do an Assassination wouldn't give you the same boost that sending a Hunter would.

Exactly they get a boost for the boring mission

Why does it matter when they will be doing the mission on their own? And how is searching old ruins boring? Isn't the Vault of Glass one of the funnest parts of Destiny?

Except in the Vault of Glass you are not sent to examine ruins, you are there to destroy Atheon.

We really don't know why we are sent there. We kill Atheon, but we have no story about the vault. We just show up and kill everything in hour.

A Warlock is more of a Scholar than anything, here is the description for the class: They’ve found a way to weaponize their curiosity, harnessing the Traveler’s light in patterns never before imagined. Warlocks are not passive scholars of the unknown. These are mystic warriors, capable of incredible acts of devastation. Titan Their lightning-charged fists will knock you off your feet. Their armor and shields make them nearly invincible. We like to think they are our faceless knights, protecting us from our countless enemies. But their power and resolve gives them the air of rising gods. Hunters Quick on the trigger and deadly with a blade, Hunters stalk battlefields as if they were hunting prey. Cunning and ruthless, they prize the undiscovered and prepare for the unexpected, channeling their powers by intuition and gut instinct. Warlocks match the description of a scholar more than any other class. Titans defend, Hunters stalk, Warlocks study. How is being curious about the unknown and learning about it boring? That's the best thing any class can do because it makes way for new possibilities.