I'll assume the question is this:

> Why measure how of the SUT's code is covered by integration tests?

and not this:

> Why measure how much of the integration test code is covered?

I think you should avoid measuring code coverage for integration tests, for two reasons:

 - **Unit tests are a much more effective way to exercise code than integration tests**.  Compared to integration tests, unit tests are lighter weight, faster, and more targeted.  Using an integration test to cover code is like using a sledge hammer to drive in a nail.
 - **Unit tests are about exercising code, whereas integration tests are about testing assumptions.**  When we write unit tests, we mock/fake/stub out the interfaces that the code depends on, and in the process we encode assumptions about how those interfaces behave.  Integration tests are where we find out whether those assumptions were valid.

Another way to put it: unit tests are about testing whether the code does what the developer intended.  Integration tests are about determining whether components work together.