Benefit of Published APIs

You definitely can create your own solutions for all these problems, but there are a lot of benefits to using applications with published APIs. One of the main benefits is that many developers know and understand what these APIs are and how they work. If you need to augment your staff, for instance, you can hire a consultant that is already familiar with these well-documented APIs. If you write your own, guess what; you also get to pay that person to learn your proprietary system.

http://msdn.microsoft.com/en-us/library/ms972319.aspx