Okay, yes, that listing does mean one is generally going to be focusing on procs, which will have a factor on the build. However, in the case of Scrappers, I'm not to sure the numbers are that far off.
For Scrappers you're putting in 2 ACC/DAM +3 HOs in every attack. After that all melee attacks can take at least 3 Damage Procs (Touch of Death, Mako's Bite, Gladiator's Strike), which leaves one Proc left to find. Now there is the one Purple Unique that can fill in one of the melee attacks if multiple of them can only take from melee sets.
So, looking at Slash. 4 damage procs & 2 +3 HOs (according to mids) is doing 262.4 damage. 3 damage procs & 3 +3 HOs is doing 256.1. So a difference of 6.3 damage. Yes, it would add up over time...HOWEVER should be noted, if one feels the ACC of 2 +3 HOs is enough, that third HO could be a Damage/EnduranceReduction for better end use. One could also just use a +5 Damage IO for 258.1 damage, so making the damage difference only 4.3 points.
And I get that one will generally not build around this style (I don't...want to for some build maybe at some point), as people generally like more survival, I do feel the list gives a good idea of what to expect for all things equal.
Now Tankers/Brutes who it says to slot 6 damage procs per attack...the list could look totally different. *nod nod*