What are valid usage scenarios for HEAP tables?
The only valid uses are for


staging tables used in import/export/ETL processes.
ad-hoc, temporary and short term backup of tables using SELECT * INTO..


Staging tables are typically quite flat and truncated before/after use.

Note that a clustered index is typically few small compared to the data size: the data is the lowest level of the index structure.

Heap tables also have problems. At least these:


can not be defragmented to reduce space on disk. This matters because used data pages will be scattered throughout the MDF for example, because data has no "order" from the clustered index
non-clustered index now point to the row, not the clustered index entry. This affects performance: Need for reaching data through clustered index with a non-clustered index


Also see


http://www.sqlbadpractices.com/heap-tables/ (Bad practice)
http://msdn.microsoft.com/en-us/library/hh213609.aspx (MS's recommendations)
http://sqlskills.com/BLOGS/PAUL/post/A-SQL-Server-DBA-myth-a-day-(2930)-fixing-heap-fragmentation.aspx (Paul Randal's DBA myths)