The problem with the Diresword is that it doesnt compliment anything else the Avengers do. dual avengercats compliment bladestorm, which compliments shooting. Shimmershield compliments defend, which compliments defense against assault. The diresword is like a crippled force weapon, in the hands of a model which is hardly a close combat specialist. Sure he has WS5, but only S3 and two attacks, (3 with the pistol and CCW) and most multi wound models are going to have T5 or better, (which he will need a 6 to wound, if indeed he can at all) AND will most likely be close combat specialists themselves. (Ogryns, Spawn, Big Mutants, TMC's and the like).
Even then, assuming the Exarch manages to wound said beastie, said beastie then gets to make any invulnerable save it might get, and must fail its own leadership test, which for most things I can think of that might fall into the catagory of potential diresword fodder, most would have at least Ld8, and many could be upgraded to better. so at best a one in three chance of failing the test. More than likely, he will only be taking one wound.
The shimmershield, on the other hand, provides a MUCH more tangible benefit by allowing the WHOLE SQUAD to take an invulnerable save against close combat attacks, which is the major weakness of Avengers. And this doesnt just mean power weapon attacks either, were talking about Rends, Monstrous Creature attacks, and possibly some psychic powers. think about it, how many other armies can boast the ability to give all of their troops an invulnerable save? and when combined with the Defend power, reducing the number of incoming attacks to begin with, and the fact that the Exarch is only a "Squad Upgrade" character, and thus can be the last model selected for casualty removal, the shimmershield makes Dire Avengers highly resilient against assault, which is good because they will be holding off your opponents assault specialists that much longer.
Okay /rant off :yes: