Why is Office for rival tablet makers so important? In the end, Office is one of Microsoft's biggest cash cows. The world is going mobile. Tablets may outstrip PCs in terms of units. And Microsoft can't just play the Windows-Office game in the mobile world because it doesn't have the market share.
Bottom line: Waiting to push Office on Windows 8 devices only just doesn't make sense.
In addition, document viewers on tablets are decent, but a real Office would be better.
You can flip a coin and guess whether Microsoft is playing offense or defense with its Office for the iPad move. Here's a look at the arguments:
Offense: Microsoft launches Office on the iPad and it becomes a bestselling app overnight (it will). This move gets folks on the iPad used to Microsoft's Metro design. Maybe Office on the iPad seeds the market for Windows 8.
Defense: Microsoft realizes that the iPad has won the enterprise with a combination of corporate buying as well as consumerization. The enterprise runs on Office day to day. If Microsoft wants to keep its corporate beachhead it has to get to Apple's platform.
Whatever Microsoft's motives may be Office for the iPad is an idea that's way overdue. There's just too much money at stake even if Microsoft has to hand Apple a cut of revenue for App Store distribution.