Microsoft cannot be profitable by keeping Office unique to Windows Phones and tablets. Now, they can leverage their platforms to make the experience better with WinPhone and Office, the built in integration with Skydrive, for example. But not having full-featured Office on every platform doesn't help Office OR windows Phone.
I agree, Office is a great product. Every company I have ever worked for uses Office exclusively. I am Microsoft certified Office trainer. I see the value. But there are LOTS of people trying to get into that space. Open Office, Libre Office, Apple's documents platform. Google docs. They are all investing in the experience. If they work better in Android and iOS (95ish % of the mobile market by the way, combined), then people will start using these competitors, because many of them don't care about genuine name brand Office, many of them just need to get a document written to ship off to the boss before the end of the day. Microsoft will erode their Office market if they don't have good versions of Office everywhere they can.