JaegerBane,
I like Crit Damage and Flank damage, actually. The thing to remember about conditional modifiers is that they multiply with your other damage. You do need to stack attack % up to a point, but once you get 20-30% it's worth looking at other options. If your weapon has 100 base damage, and you have 20% attack, you'll do 120 damage. If you flank, you'll get +25% of your modified damage, so you'll hit for 150. Suppose you have the choice to craft attack% or flank% (obviously within the same recipe you never do). If you add +10% attack, you now hit for 130 normally or 163 on a flank. If you craft +20% flank, you still hit for 120 on a regular hit but now your flank attacks do 174!. If you make just half of your auto-attacks from flanking, you break even. Now consider your activated abilities -- If you make sure to always use them from behind, you're going to get huge value from the flank mods. Flank mods are a no-brainer for dagger rogues, who have stealth and flank attack to get behind and already need to flank to exploit twin fang. If I ran with two warriors and no Varric, I would put flank bonus in all my leather slots on my off-tank, and use charging bull.
Crit damage is harder to value. For pure DPS, it's better to build crit damage than crit chance whenever crit chance is >1/2 of crit damage. Base damage is only 40%. Base crit is 5%, maybe +5% from Varric, plus incidental cunning bonuses. Hitting 20% is virtually guaranteed for rogues and happens surprisingly often for warriors. The hard part is that crits aren't all about damage. Warriors usually care more about activating passives than multiplying damage. The crit-linked debuffs last so long 20% crit is plenty, but the CDR stays good until you run out of stamina or animation time. A 2-handed battlemaster warrior would probably never build crit damage. On the other hand, rogues have various passives that can give auto-crits or massive crit chance in the right conditions. If you're team is set up to rely on those, then crit damage is great.
TLDR: Flank damage and crit damage get better the more attack% you already have (and crit damage also gets better the more crit chance you have). Most rogue builds should consider crafting one or both of these stats.