I also learned recently that if you cancel half a round trip the miles are not redeposited until the other half of the trip is flown or canceled.
I'm not sure I follow. If you change from a round-trip to one-way award prior to travel, the refund should be immediate. If you change it after travel has begun, the rules do not allow for a refund at all. I can't think of any situation where you'd be able to get a refund but not until flights had commenced.
The only reasons remaining to fly a round-trip award are (a) sometimes the price is lower, as
fumje mentions, and (b) the Excursionist Perk requires it.