Realistically...
For most of us it's not so much as to whether there needs to be alternative versions of Microsoft software available for ARM, but more whether the licensing models will cover installation on any platform.
If Microsoft decide to get greedy and start saying that ARM-based devices require a separate special license for software like Office that aren't covered already in volume licensing and/or retail licenses then people aren't going to shell out for it.
I hate to say it, but I'm half-expecting Microsoft to discourage local installations of Office for tablets anyway, and try to push Office 365 on everyone. They do tend to have a certain tendency towards blindness to the fact that most people out there don't have always-on, always-active high-speed internet connectivity that never fails.