Maelwys

As a Katana/Regen Scrapper, I have Divine Avalance slotted as 3 Damage Resistance, 2x Accuracy and 1x Recharge Reduction. This is enough to make sure that it hits consistently, and it overlaps (can get a _very_ brief overlap of 3 Divine Avalanches with this build with +3 SOs, two DAs can be up constantly).

Stacking two such Divine Avalanches increases my defense dramatically (about 48%), however it does reduce my Damage Per Second by a large amount, due to it's considerable execution time and relatively low base damage. For that reason it is inefficient to use it much in groups.

It's a great soloing aid though. With a Recharge SO in Soaring Dragon and Golden Dragonfly (Katana's two heaviest hitting attacks, Knockup and Knockdown respectively) I can chain DA-SD-DA-GD more or less constantly, without Hasten running.

For I6 I think the defense boost is fine because of the DPS tradeoff. I'd say it's about as useful to Katana users (and presumably Broadsword users) as Touch of Fear can be to Dark Melee scrappers.

(Hey it's on the primaries, so SR scrappers can get it too if they want!)

Assuming a smashing/lethal attack hits you for 200 unresisted, according to those numbers you would currently resist 99.4 damage.

So now that's 100.6 damage incoming. With Tough running you would resist a further 35.2 damage, which effectively shaves 35% from the 100.6 damage you would otherwise be taking.

It's really up to personal preference whether you think the slotting of two pool powers is worth that percentage difference... I'm currently debating whether or not to take Tough on my katana/regen. This would mean dropping one of (hasten/resilience/final epic pool power) from my planned build to make room for that damned filler attack that I'll never use...

However conventional wisdom would say that tough woud be a better addition to your build than a SR/Regen scrapper's build... adding tough would currently cut 35% damage from incoming attacks for you, whereas it would only shave off the base 17.6% for someone without any other res buffs.

If you're worried about endurance issues, one thing to think about is epic pools.
At level 40 you'll be able to take Conserve Power soon! ^^