I use safeguard/vene for several reason and occasions.
On pretty much all bosses where there's a tank taunt/switch, i taunt+sb+slam+vene, thus dropping 10% threat from other tank, making it easier for healers to heal him(-15% dmg reduction) up and focus on me
The -15/30% damage reduction on a 30 second cd is pretty powerful, in alot of scenarios this tier
Depending on how you execute the fights here is a couple scenarios where i use vene.
1. Rag taunt switches and during add phase.
2. Beth following a widow/to reduce damage taken.
3. Alysrazor if you tank switch.
4. Baleroc emergency help on decimation blades.
5. Domo during schytes i pop it regulary to help reduce overall damage.
6. Shannox last phase to help "MT".
I mean, i could make the list longer then this, but i think i proved the point
OT: Thork looks good, nothing to improve on really :P