You're not totally correct concerning a HEAP, some rdbms allow the table and index structures to be defined ( not all B Tree's by default )  heaps do not always fragment. IF the table is not subject to updates which cause deferred updates and there are no deletes then your heap will not fragment, all new data arrives on the top of the heap - hence the term. Secondary indexes on a heap can be defragmented without issue. Technically if your heap is a fifo ( first in first out ) then even the deletes would not fragment the table structure. 

I agree that it's best practice to have a clustered index, and in fact most tables I see are still heaps , but with a clustered index. Also agreed that the leaf or data fragmentation in a table cannot be removed without a clustered index, however not all heaps fragment.