Lately, there has been a lot of talk about what's working and what's not when it comes to building the first instances of a SOA. Reuse seems to be a key component, with the most of those implementing a SOA pointing reuse out as a key driver. I think this trend will continue for the next few years, as those within enterprises stand up a SOA for the purpose of building a library of services they can abstract from system to system.
However, agility, or the ability to create an agile IT architecture that's able to change as the needs of the business changes, is also a major benefit, perhaps more valuable then reuse in the long run. The issue is that in the short term is there is really no good way to measure the value of agility...it's more of a long term notion.
Thus, why many are seeing the benefits of a SOA through reuse of services, longer term it's agility that will pay the rent for most SOA implementations, and is really what makes the approach strategic to modern businesses. My point: Agility should always be in the back of your mind when building out to a SOA, more so than reuse.
I believe that agility is just a second-order effect of reuse: that library of reusable services is what provides the agility. In that light, they're really the same thing.
Dave, I agree, but would even emphasize it stronger by stating "agility" should be in the "FOREFRONT" of your mind as a SOA business case driver and that "reuse" should be a required design tenet that will naturally insure agility. If we design for reuse, by definition we are delivering agility.