There are situations when executing build phase of a job marked with exclusive execution should not wait till all currently running jobs have finished.
E.g. imagine you have a planned backup of your SCM server. You have typical tasks that download projects from SCM and compile them and run tests. Execution of these jobs can take very long but it is unlikely they fail once they have started (SCM is queried on start). If you want to avoid any other job to start running if SCM backup process starts you can schedule a dummy job marked with exclusive execution that simply waits till SCM server is back online in its build phase.
The problem with the scenario described is that if currently running jobs take very long to complete, actual check on SCM status will not be performed until such long execution has finished. It could happen that those long-running tasks finish way after SCM server is back online, so Jenkins execution time is wasted (no jobs are run due to dummy job still in pre-build phase).
For these an other reasons IMHO it would be great if waiting for running jobs, when enabling exclusive execution, could be skipped by configuration.