As you've probably figured out, hpoj was the predecessor of hplip, so you if you have an all-in-one printer, you need either one or the other, but not both. If you have a standard printer, you can use either cups alone or cups+hplip.
You're right, even after loading the driver you need to keep the hplip extension loaded because the driver uses the hplip hp backend instead of the cups usb backend.
There are a few benefits with hplip (even if they don't add up to 5MB) like hp-levels, hp-align, etc.