A transient replica transitioning to full and vice versa doesn't cease to be the kind of replica it used to be before. I think this could cause availability issues if a node goes down and it could even cause unavailable with some ring movements with all nodes up. I haven't convinced myself 100% this is true, but I think it should be possible to construct an example.
We can't read from pending replicas so if a ring movement were to make 2/3 out of a group of 3 pending we would have this problem.
The key difference here is that the existing replica should be available during this period as the kind of replica it was before. A full replica remains full until nodetool cleanup is run. A transient replica remains transient the entire time.
Obviously this doesn't hold for transitions to/from not replicated at all.