There are only 3 (objective) reasons to full clear a rift:
1. You have a lack of keystones and want to maximize the Rifttime per Keystone
2. It's a really good density rift
3. You/your group is so fast in clearing a rift that the 30 seconds between 2 rifts would grant you on average 0.4 legendaries (100 bloodshards), if you killed mobs during that time.
There's still the option to close the rift and go back in to continue your killing till time runs out though.
So basicly the only REAL reason to ALWAYS fullclear a rift is a lack of keystones. This means if you already got your perfect ancient RoRG, you can always fullclear. If you don't, you need to run bounties anyway and therefore shouldn't have a lack of keystones.
Ofc there's always personal preference, but I was only talking about the objective point of view.