I am being told that there is no benefit to having a primary key defined, especially a surrogate key of identity if it is not used in joins. 

We have another table of account information that does have unique rows (key is account_num) however the uniqueness is enforced only by an index that is created after the data is loaded for the day. 

The most common use of the monetary transactions is to match with accounts by joining on account number.  Obviously this will return all the payments for an account for that day.  Since we join on the account number (a good candidate for an index) the developers (and my manager) claim that the surrogate key created by identity is of no value.  They insist that their experience outweighs any academic discussion about relational theory and all tables being implemented in 1st normal form minimum. 

What would be the easiest way to demonstrate that performance is bad when a table is a HEAP?

Thanks for you comments,

Glen