In Fall 2013, when Apple released Pages for IOS v2, Pages v5, and updated Pages for iWork beta to be compatible with the other two — that was Apple sending their users notice that they were done with Pages ’09. Though Pages ’09 continues to work for the most part (Share documents to email is now broken) on Yosemite, there is no assurance, or Apple consideration, that any future operating system release will not break it.
As with ClarisWorks/AppleWorks documents, one also needs an active Pages '08/'09 migration strategy to MS Office document formats. This will provide forward document compatibilities and alternative application choice. There are AppleScript solutions available in the Pages community that allow bulk Pages '09 to MS Word document migration.
ClarisWorks/AppleWorks documents can be opened in the free LibreOffice, itself an MS Office replacement suite.
MS Word is what it is — all the functionality that is missing, or better implemented, or easier found help — than offered by Pages v5. There is a reason that internally, Apple uses Office for Mac 2011 — no lost productivity, or document compatibility issues. Hitching your future document plans to Pages v5, is in my opinion, a revisit of Apple's history of Office suite abandonment, and islands of incompatible documents.