As for grind, well, I really think that remains to be seen. You look at games like Aion where you repeat the same instance over and over and over until your fingers bleed, and because you're relying on luck for the armour set piece to drop you could be there forever. Guild Wars' way of getting around this is to allow you to buy your gear through a token system, so even if nothing useable drops for you in the dungeon, you still get some progress toward being able to afford something you want. Given the calculations in the OP are correct, I don't really consider this real grinding, certainly not on the scale of some other games.
Being realistic though, you do need to work for gear sometimes. If it's just given to you on a plate it's no longer special at all soooo I guess this is a balance ArenaNet need to make a gamble on and we will see if it pays off.
Problem is the only way to get a full dungeon set is to replay the dungeons 80+ times which is steep. More tokens should drop and there should be a random chance (if only 5%) that you get an exotic item from at least one boss. Putting people off of running dungeons won't increase longevity of the game, quite the opposite.