First off, special melee attacks don't have some arbitrary 5% bonus to their accuracy.. It's a specifically listed 10% higher base hit chance. The reason there is a difference between Resistance Chance (applied to Force/Tech attacks) and Defense Chance (applied to melee/ranged attacks).
Once again, there is a *difference* between Defense Chance and Resistance Chance; having one does not equate to the other. NPCs are known to have a Defense Chance of somewhere in the region of 6-8% but to not have one whit of Resistance Chance as evidenced by the fact that Force/Tech attacks *don't* miss while melee/ranged attacks, even with a bit of accuracy, *do* miss. Blade Storm doesn't miss because it's a Force attack (100% base accuracy) and NPCs have no Resistance chance (0% chance); combine those 2 aspects and you get a 100% hit chance without every stacking a point of accuracy. Special Melee attacks miss because they are special melee attacks (explicitly listed in your character tab as having 100% base accuracy compared to the 90% base accuracy of a basic melee attack) and NPCs have a known Defense Chance (in the 6-8% range); even including the ~4% accuracy, those 2 facts combine to get you a roughly 2-4% miss chance. Keyboard Ninja's 5% miss chance number comes from 6% defense and 1% legacy accuracy. It's not some arbitrary 5% additional accuracy for melee attacks.
In terms of the 5% boost on special attacks, here is exactly what KeyboardNinja wrote.
Note: Double Strike and Spinning Strike are both "special attacks". Among other things, this means that they get a bonus 5% accuracy boost. This is why they have 96% accuracy overall.
Mechanically, 5% accuracy boost and 5% defence chance is the same as 10% accuracy boost and 10% defence chance. Regardless, the point remains that close to 50% of my damage as a Guardian is subject to missing. Admitted its *ONLY* 42% of the damage once I'm into the 'normal rotation' phase but that is still quite high. 4 of my 7 highest damage/threat attacks are all subject to accuracy.
Again, I am not saying accuracy is good (nor needed) for a tank. I was saying that accuracy is a good way to increase threat if you are having trouble as a Guardian.
EDIT: Perhaps I should just have worded it differently at the start. If you need more threat as a Guardian: make sure you put points into accuracy before you bother adding points to other DPS stats. Is that a more agreeable point?