Don't under-estimate warding staves. They are so good that I normally include 2 in my purifier lists. Why? Because it lets me tarpit enemy assault units that I normally should have no business being in assault with. Units like monstrous creatures, necron overlords, enemy terminators, characters, etc. And it is this tarpitting feature that has won me several games when without it, those enemy uber units/characters would have torn through my grey knights.

You can't always rely on teamwork to kill off the enemy, especially when its your fragile 5-man unit that gets assaulted first. Or oftentimes, your units are already weakened by enemy shooting.

And remember, that 1 surviving warding stave guy can still possibly do a lot of damage with Cleansing Flame each and every turn he survives.

Finally, warding stave can save lives through different initiative attacks. I'll give you an example. My GK terminators got charged by Logan and his wolf guards. Logan does damage, of which my stave saves 1. Then the lightning claws do damage and my stave saves another 1. Finally, the thunderhammers do damage, which my stave saves yet again. It should have been a wipe-out of my terminators, but instead, I still have a few GKT's left to kill more wolf guards with.