That doesn't make sense, though.
Consider my use case: We buy products from Farmlands that are mostly Horse Feeds, so I have set up the default account code for Farmlands to be "Feeds". However, occasionally we buy other items from Farmlands that are not feeds. I can't create Items for those occasional things with a different account code, because that account code is going to be ignored. Why would the default account code for the supplier override the specific code you have put against the Item?
Using your solution, to achieve this I would have to remove the default account code for the supplier, and then either define the account code for every line in a purchase, or define all the different types of horse feeds that I can possibly buy from Farmlands as items with the "Feeds" account code, as well as the occasional items that are not horse feeds. That's a lot of setting up just because you've decided to go against the way that most other software products work.
The MYOB approach means that you have to spend a lot more time 'correcting' the account code on each line of the purchase by either selecting one every time (if the supplier default is not defined), or by 'correcting' the ones that do not match the default. The latter may lead to an increased risk of human error.