I have noticed a number of weapons held both by adversaries and available to deathwatch marines have the shocking quality. With my current understanding of how that quality works it seems to me to be completely and utterly useless. Space marines have on average 40 toughness and at minimum 8 armor to every location. So thats +80 bonus to the test(reduced to +60 due to the rules on max bonuses) to the toughness of 40 putting you at 100 so making it impossible to fail. In addiction space marines possess unnatural toughness so the difficulty is reduced even further. Anything with such a low toughness and armor combined that could be realistically affected by a shocking quality weapon would be complete cannon fodder anyway. Is there something I'm missing with the rules?

I think you're right on the money - as far as the PC group goes. The only time I've seen a Space Marine affected was a Psychic Scream by a Hive Tyrant that got a Space Wolf (no helmet and a -30 penalty to his test to resist stunning). Shocking is lethal for taking out many NPCs; it's a good way to kill/incapacitate some squishy characters that the team may want to protect, like the local informant who has information the team desperately needs but has been captured by the group of CSMs operating on the planet.

It has its role, but it definitely isn't a serious threat to your average Space Marine.