Its a simple math problem:
3/5 Opportunity gives you 12% on your Backstab.
5/5 Opportunity gives you 20% on your Backstab.
3/5 Lethality gives you 18% on all criticals.
5/5 Lethality gives you 30% on all criticals.
Spec 1:
Assume you are at 25% critical rate, if you are 3/5 Opp and 5/5 Lethality, then on you deal 112 for Backstab at 45% of the time and 291.2 at 55% of the time which translates to on average 50.4+160.16 = 210.56
Spec 2:
If you are 5/5 Opportunity and 3/5 Lethality, then you are doing 120 at 45% of the time and 283.2 at 55% of the time which equals 54+155.76 = 209.76
Note the lack of units on my "damage" numbers, for instance 112 means 12% more of normal (100%)
Pretty minor differences, this however changes dramatically once your crit rate changes. Losing say 5% crit for hit would translate to about a loss of 9 and a loss of 8 for specs 1 and 2 respectively.
The real reason that Combat Dagger specs suck is not because of how fights turn out (mobility and etc) its how the Rogue class works in terms of hit, our hit cap is extremely high and rightfully so. So in order to maximize our primary source of DPS (white/normal weapon strikes), we have to prioritize +hit first, however +crit tends to share/conflict with +hit on gear. This conflict is plenty apparent in our gem choices. White strikes also benefit more from AP than crit as our talents tend to only buff special critical strike damage. To digress further, this is apparent in the post-Hemo buff era, where we can afford to give up the bottom half of the Combat tree for the top half of the Subtlety tree essentially trading Special DPS for White DPS.
So the inherent problem in Combat Daggers rears its ugly face, you have itemization issues and your damage is **** poor if you don't land a critical. The big issue is how to not gimp your white damage and still maintain a respectable critical rate.