Even from Windows7, I've been using the Office web apps via my SkyDrive almost exclusively. No real issues that I've run into and I think most people should be doing that on a Surface(Poor messaging by MS). The full Office is there for power users IMHO.
That said, its desktop mode Office because they are emulating(or just ported) the old Win32 libraries used to run old office code. This is likely the cause of the CPU spike.
Windows (NT) has been cross-platform from day one, so there wasn't any need to emulate anything. The kernel, Win32 and everything that ships by default with it ran on at least x86, x64 (Itanium), x86-64, DEC Alpha, PowerPC and others. So yes, it was likely ported (but I doubt there was significantly more effort than just rebuilding everything, given that the code shouldn't be that unportable to begin with).
That said, its desktop mode Office because they are emulating(or just ported) the old Win32 libraries used to run old office code. This is likely the cause of the CPU spike.