The only time I move from a blowoff to an airlock is if I'm sure primary/potentially explosive fermentation has passed, and:
-I need to move the fermentor out of my fermentation chamber
-It's time to bottle! (duh)
And, most most importantly:
-I'm going to cold crash
Cold crashing your brew will cause it to take in quite a bit of atmosphere as pressure increases with the temperature dropping. If your blowoff tube is still in a jug of starsan, it might pull all that right into your beer! S-shape airlocks are great for this since they work in both directions.