Can an onlooker even learn anything interesting by de-anonymizing a transaction? If people cannot see what exactly the items purchased are but they only see the transaction of funds in the blockchain, does anonymity therefore even matter?

Unless you get associated with criminals later if their address is seized and your transaction to them is viewable on the block chain?
Knowing what you bought isn't the only information that you can get from a transaction. For example, you could possibly discern:


The transaction creator's salary. (If the inputs in a transaction came from your salary payment.) At the very least, you put a minimum on how much money they have.
The transaction creator's location. Many websites track where payments come from, geographically.
Where you buy things. If an onlooker has access to address information from one party, say Overstock for example, then they can see if you transact with them. And sometimes where you buy things is a strong indication of what you're buying.


So, knowing exactly what item you bought is not the only information that a user may be trying to keep private.