The software community is currently on a big standards kick. If you've developed something, you try to get it standardised (example: Microsoft pushing the CLI through ECMA and ISO's fast-track process). You then criticise everyone else for 'not following the standard' or for 'extending the standard.'
I don't actually care much about standards. They're useful, yes, but I'll use a non-standard product if it's better. There are two standards for the SQL database query language, SQL-92 and SQL-99. Most database products now support a subset of SQL-92; newer products are targetting SQL-99 (Microsoft's next release of SQL Server will have some SQL-99 features).
Can you produce a useful database application using only SQL-92 features? Possibly. Can you produce a better application using your vendor's proprietary extensions? Almost certainly.
POSIX is the ISO standard for interfacing programs to an operating system, and for presenting programs to a user. Which major operating system has virtually no POSIX features, and yet has over 90% of the installed base? The one that makes sense. The one that has richer APIs. The extent of POSIX support when programming to Win32 is that some of the POSIX extensions to the C run-time library are available - with a leading underscore - in Microsoft's C run-time. Compare _open to CreateFile and you'll get just a flavour of how much more Windows offers to the developer.
As developers, we have to weigh up whether following a standard is beneficial to our users: either in terms of being able to replace our software (erk!) or interoperate with other software. The downside is that it may be difficult to follow the standard, or it may simply be overwhelmingly complex for the particular application.
Microsoft have been criticised recently (i.e. in the last six months) for both the new Office 2003 XML schemas for Word and Excel (a lot of complaints emanated from Sun^H^H^HOpenOffice because Microsoft didn't use their schema) and for the WinFS schema language - why didn't they just use XML Schema?
In the first case, features of Word and Excel simply didn't map onto the OpenOffice schemas in any reasonable form. And in the second, XML Schema is simply too complicated, and doesn't match the WinFS object model. With this much mismatch, the programs would probably contain more code and run slower than they could have, leading to unhappy users. In the former case, OpenOffice's schema is probably a good match for their internal object model - but it doesn't benefit Microsoft.
Take the flip side: when Microsoft were looking for a better authentication technology for Windows 2000 Active Directory domains, they could have invented their own. Instead, they went with Kerberos. Why? Because it had the features they were looking for (a distributed authentication system, without needing to centralise authentication to a single group of servers) and was already known and trusted. However, it didn't have the ability for the administrator to change a user's password remotely. So Microsoft added that feature to their implementation of Kerberos.
Sun, as always, cried foul! Our Kerberos implementation (or, rather, MIT's) doesn't do that. MS must be making their client work best with their server - let's complain to (in this case) the EU.
But who benefits? The users do. The administrator doesn't have to open a remote command shell on the login server to modify the password file directly. Computers can add themselves to the domain, generating their own passwords (in Windows domains, computers have accounts as well as users) without requiring the administrator to explicitly set one on the login server, then enter it correctly on the computer.
It's helpful to control both ends of the connection, because you can make more extensive changes without reference to others. But I don't believe that Microsoft are deliberately trying to disadvantage their competition - they're just looking for ways to improve the user's experience through technology.