We struggled with the same issue, and decided to replace Outlook desktop with Outlook web for the 600 desktops that we are migrating to Linux and LibreOffice. OWA in exchange 2010 is robust enough to address the majority of our enterprise needs. We also found that cross-platofrm compatibility with OWA is much better than desktop clients and prefer OWA on Mac, Windows, and Linux, to a mixed back of clients on each OS. You're right that MS prevents licensing portions of the office suite separately, as we had considered keeping outlook desktop clients, this is annoying to say the least, and played a big factor in our deciding to boot MS Office and windows from many of our desktops altogether.

I think this depends on organizational perspective. Mine was that 1M USD to license Windows and Office every three years is not chump change. So we are moving approximately 600 desktops to Linux and LibreOffice. As we move our workforce increasingly towards web based systems and workflows, desktop tools in the MS Office suite rapidly decrease in value. Similarly the value of windows as the default corporate OS is also rapidly decreasing as we look to cross platform solutions where we can work from next-gen mobile devices and tablets. Saving ~330K USD per year and reinvesting a small portion of that savings into tool improvements and customizations that can be shared with the community sounds like a win-win, and a morale boost for our internal dev teams. Sure we would never make a business decision to replace something that works now, with something that doesn't. But Libre office works well enough now (we tested), and MS office has plenty of its own challenges and limitations. These tools working best together is also subjective. There are better document management systems than share point in the marketplace, and evolving standards such as CMIS for content interoperability between systems that Sharepoint now supports. Microsoft's lock-in strategy has been a double edged sword, as focus on making sure that their systems work better with each other has been counter to ensuring interoperability with other systems in the enterprise. For large enterprises Microsoft systems to not represent the majority.