Does the inability to spend only part of a transaction result in "too many" extra transactions?
Why does the receiver bother sending back change?


This assumption is wrong. Nevertheless, you are correct saying, that you need to spend the whole transaction input (UTXO) balance on your address, but the person who makes the change is the sender, not receiver.

Example, You have unspent balance (UTXO) of 1 Bitcoin and you want to pay 0.2 BTC for coffee. You create a transaction spending 1 whole Bitcoin to two separate receivers. 0.2 BTC (tx output) goes to merchant and 0.7 BTC you direct back to your address (the change). The difference between UTXO and sum of tx outputs is called fee, in this case 0.1 BTC.


  Does the inability to spend only part of a transaction result in “too many” extra transactions?


No, since it is still 1 transaction with just two output in case there is a change. No extra transaction is made.