You are right but that itemization on ToT weapons either has expertise or hit and you dont need any inside proving grounds pretty much so its better to take garrosh daggers that have PURE agility sockets + 120 agility bonus (tot daggers only have +60 agiilty bonus) and also has +crit instead of hit/expertise.
The extra 60 agi from the socket bonus on the heirloom weapons are at the expense of 80 on the weapon itself. Hit and expertise is most certainly needed until cap (probably a 6% cap for PGs, never really noticed mob levels in there, I gear for 6% caps in CMs anyway). You will end up with more agility using fyn's flickering dagger than you will with hellscream's razor, and you'll have more secondary stats too. Hellscream's razor will give you 665 agi, 578 stam, and 346 secondary stats. Fyn's dagger will give you 685 agility, 498 stam, and 581 secondary stats. Megaera's fang will give you 605 agi, 498 stam, and 578 secondary stats. Is 60 agility really worth 240 secondary stats (generally, no, your best secondary would have to have 1/4 the EP of agility for that to be a worthy trade off, that's usually where white hit--your worst secondary--is). For completeness, iron qon's knife would give you 685 agi, 578 stam, and 408 secondary stats. What it comes down to is this: the prismatic socket is extra stats. Free extra stats, at no cost.