Assuming you have build plans which require unique access to shared infrastructure at a given point in time (such as shared development databases, for which you can have multiple plans which access the same database, but not at the same time), it would be useful to have an option to state that only one of certain plans can be running at the same time.

The basic use case here is distributed test runs. Assuming that your basic plans are:

• Build (exactly 1 plan)
• Test (10 plans for testing in different environments and with different configurations, but which have to be run serially)
  You would want completion of the Build plan to schedule all 10 Test plans, but you want to make sure that no matter how many build agents you have available, only 1 Test plan runs concurrently.