DBCC FREEPROCCACHE along with Re-Organise Index maintenance task While creating Index maintenance plan in SQL Server, they have also added execute T-SQL statement DBCC FREEPROCCACHE. Is this beneficial, or are there any side-effects?
The DBCC FREEPROCCACHE command is not beneficial (for index maintenance), and should not be executed in this context.

It flushes the plan cache, which causes all query plans to be recompiled when they are executed again.

If there's no way to turn that setting off manually, it is better to use recommended third-party index maintenance scripts, such as those by Ola Hallengren or Minionware.

EDIT: For an index re-organize, you should add a step to update statistics as well, because they are not updated automatically. This step alone has the same effect as invalidating stale plans, without the negative of flushing the entire cache. The above third-party maintenance solutions include a step to only update statistics that need updating.