What do you mean by "regressions"? I'm unsure if Pivot tables is supported in MacOffice either.
Why Windows? For me:
ActiveDirectory
SQL Server learning & certification
Exchange 2007/2010 learning & certification
WinXP/Win7 learning & desktop support technician certifications
Citrix XenApp v11, Citrix Server, Lotus Notes (but v8.0.1/8.5.1 both work just about exactly the same in OS X Panther & onward.
Just about holding a job, refining skills to get a new job; and earn enough money so that money can work for me & itself so that my kids can do what they wish: daughter is an aspiring artist & hermit, son wants to be a comedian (don't ask, its all Jim Carrey's fault)!