I saw this on CNet this morning, an interview with Steve Ballmer of Microsoft and Ron Hovsepian of Novell, talking about the recent intellectual property deal MS and Novell signed.  I thought this quote in particular, from Ballmer, was a good succinct statement of why Microsoft pursues interoperability:

I know our customers have Linux. It's not my job to accept or not accept. I'm going to try to tell them every day why Windows is a better choice, but when they make the choice to have Linux, I want it to interoperate with Windows. I want to provide that value, I want to use that interoperability as a source over time of other opportunities.

In other words, Microsoft doesn't pursue interop because "it's nice to do."  It's not pro bono work.  It's not "out of the goodness of our hearts."  It's a business opportunity.  Microsoft wants to service our existing customers, and we want to reach out to new customers.  Current and potential customers tell us they want interop, so that's what we try to deliver.  In lots of different ways. 

Companies that talk about "open standards" as their philosophical foundation aren't being completely clear about their motivations.  If it's a publicly-traded for-profit venture, then the goal of the company is to make money.  That's the way the world is.  "We embrace open standards" .. or ... "We do interoperability"  - these are not better, more upright, or more admirable positions for a company to take, as opposed to any other position.   Supporting standards and investing in interoperability - these are means to an end, an approach to making money.

[added 2006 December 12:]More and more software companies are doing interop these days, more effectively, because of appropriate and consistent customer pressure.  Customers are demanding interop across a number of different axes, and companies, including Microsoft, are trying to deliver.  We think we're on the right track and we think we're getting better here, and we welcome feedback.

So the pursuit of interop and the embrace of standards by companies are not altruistic actions. They are attempts to deliver the value customers want, and make money from that value.   At the same time, it is disingenuous for representatives at IT companies to say "X is proprietary and closed," as if that alone damns a technology.  It says here there is nothing inherently wrong with proprietary technology.  It is part of what fuels the industry.  The Intel x86 chip is proprietary Intellectual Property.  The AMD Opteron includes proprietary IP.  Oracle database.  IBM WebSphere. BEA WebLogic.  Microsoft Windows.  [Adobe] Macromedia DreamWeaver. Certainly there are non-proprietary technologies too, that have contributed to the industry, but it is not a case that one flavor of IP is inherently "better" than the other.   

To say "X is proprietary" as a derogatory or dismissive comment is throwback language. It dates the speaker to an era in the industry when interop wasn't really possible, when hardware and software platforms were inextrcably linked, when many vendors vied for business and often went belly up, stranding customers. We've moved on as an industry since then, though the rhetoric hasn't completely caught up. [end of edits]