Also, lets not forget they are classified as exotic not power weapons. They are both balanced and defensive because that's what they are, and they require additional training to use. Not to mention its attached to the arm freeing up the hands.
Additionally, why wouldn't the two qualities stack. If you look at the mordian pattern power sword in the core rules it is balanced and provides an additional bonus to parry, without being defensive and suffering a penalty to attack, as well as presenting a superior damage profile.
Should those bonuses not stack, since it makes it arguably better than the forearm powerblade? 15 to parry vs 25 with no penalty to attack or need for additional talents.
Well, my initial feeling for why they don't stack is that they both give you varrying degrees of the same ability: being able to parry easier. The mordian-pattern specifies that it is Balanced (so takes no penalties for attack) but grants a +5% additional bonus to parry. If you feel the abilities do stack, then you can't pick and choose: an item that has both Defensive and Balanced (like these blades) would have +25% to parry but -10% to attack also. The Mordian-Pattern is +15% to parry and no attack penalty.
If your character is a melee specialist (especially if you have counter-attack), it's likely the Modrian is a better choice than the Exotic blades. On the other hand, if you're a heavy weapon specialist who wants a defensive edge when you get drawn into melee, the Forearm blades make a great choice because they keep your hands free. They also seem like a good choice for someone with poor Dodge and WS who's looking for a way to give themselves a boost. Or someone who fights with pistols in melee.
The argument that something is classfied as Exotic and should therefore be' better' is great in theory but isn't always borne out in the stats in the books and isn't universally true in all situations and for all characters. Is a Digi-Las 'better' than a Hellpistol? A Needle Pistol? It really depends on the situation and the objective.
If one can mount a range weapon to their forearm then as this weapon demonstrates one can certainly mount a power weapon.
Except for the fact that RAW only Pistol weapons can be forearm mounted, of course.
I used to want a forearm powerblade, but settled for a retractable powersword mounted to my arm like the predator.
Funny, that was how I imagined the forearm blades.
Does sound pretty cool. I should invest in something like that for my Moritat in DH. Though I'm afraid I'll be too much like Ezio from the Assassin's Creed games.