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At the Battle of the Five Armies, it seems the Orcs would have had much more success if they had waited for the Dwarves and Elves and Men to battle each other before attacking.
I am speaking to the movie, have not read the book, but if the question applies to both, please feel free to answer both.
In the book, the blossoming conflict between the dwarves and the combined armies of the Elves and the Men of Dale is halted by Gandalf, who alerts them to the approaching orcs (referred to as "goblins" in the text):
Bows twanged and arrows whistled; battle was about to be joined.
[...]
"Halt!" cried Gandalf, who appeared suddenly, and stood alone, with arms uplifted, between the advancing dwarves and the ranks awaiting them. "Halt!" he called in a voice like thunder, and his staff blazed forth with a flash like the lightning. "Dread has come upon you all! Alas! it has come more swiftly than I guessed. The Goblins are upon you! Bolg of the North is coming. O Dain! whose father you slew in Moria. Behold! the bats are above his army like a sea of locusts. They ride upon wolves and Wargs are in their train!"
The Hobbit Chapter 17: "The Clouds Burst"
The combined armies (all of whom hate the orcs more then they hate each other) then set a trap, attempting to lure the goblins into the valley:
On the Southern spur, in its lower slopes and in the rocks at its feet, the Elves were set; on the Eastern spur were men and dwarves. But Bard and some of the nimblest of men and elves climbed to the height of the Eastern shoulder to gain a view to the North. Soon they could see the lands before the Mountain's feet black with a hurrying multitude. Ere long the vanguard swirled round the spur's end and came rushing into Dale. These were the swiftest wolf-riders, and already their cries and howls rent the air afar. A few brave men were strung before them to make a feint of resistance, and many there fell before the rest drew back and fled to either side. As Gandalf had hoped, the goblin army had gathered behind the resisted vanguard, and poured now in rage into the valley, driving wildly up between the arms of the Mountain, seeking for the foe. Their banners were countless, black and red, and they came on like a tide in fury and disorder.
The Hobbit Chapter 17: "The Clouds Burst"
They're playing on orcish psychology: tease them with the easy kill, and then lead them into a trap.
The reasons aren't explained in the film, so we have to speculate a bit.
It's possible that the timing just didn't work out for them. We don't know how long it takes the were-worms to tunnel through the mountains, but it must take some (those are pretty thick mountains) and, though we don't actually know how they were controlled, it seems unlikely that precision manoeuvres were practical. Considering Dáin's dramatic entrance, it's possible they either didn't stick the timing, or else they decided not to bother with the fancy strategies and just go all in.
And, in fairness, the extended edition reveals that they did actually wait a certain amount of time before they broke in, though certainly not as long as they might have:
They may have expected the Elves to stand back and not get involved. This wouldn't have been a terribly wise assumption, given the history between the Orcs and the Elves, but it's also not entirely without merit - Thranduil's forces to wait quite a long time before engaging.
They may have been counting on their superior numbers, which they certainly had; they're able to sustain the battle on two fronts (in the valley and in the city), and press the defenders into quite a difficult position. It actually almost worked for them
