The best use I've found for the Watcher's Guide is to give it to, say, Wyll, or your main character if they're a Warlock. It means if your Eldritch Blast misses, you're more or less guaranteed a hit the following turn.
The Guide (like all spears) has its uses as a weapon - not often, but sometimes, the enemies line up perfectly for a Rush. More mobile characters like Lae'zel and Wood Elves benefit most as they are most likely to be able to get into the right position. (Get hold of Crusher's Ring early to maximise utility.)
The other thing to remember is, if your movement doesn't quite get you close enough to strike an enemy with a melee attack, you can equip the Guide (or any spear) and use Rush, because the attack gives you extra movement. It basically adds 9m to your movement, with the condition that you have to attack at the end of your move.