Honestly, it varies.
I have MS Office 2010. That's going to last me quite a while I imagine. Going to LibreOffice when I've already got MS Office is going backwards - the only thing it gains me is native use on Linux, but then there's no motivation to move if what I've already got is the best product out there. If you're talking about having to buy future versions of MS Office to support newer formats, well... so be it. It's extremely unlikely LO will support them in their entirety and I'm not begging for money anytime soon.
Marginal productivity is cumulative. I make a decent salary and am prepared to use that to make my life more comfortable and less stressful. Absolutely worst scenario - I'd pirate. I'm very good at knowing how to pirate safely over years of doing it as a kid. I'd prefer not to for moral reasons, but I'll never settle for anything less that the best (functionality wise) product out there. It's not worth using substandard software.