We are talking about situations where there was't enough space on the SSD.
So the alternative from copying a 100 GB game within 10 - 15 minutes from HDD to SSD would be reinstalling 50 GB from disc (30 - 60 minutes) plus redownloading lots of GB patches from Sony's servers (hours).
True, hence I'm not planning on buying more (active) games than can fit on the SSD. I've spend too much on games I never even play.
Plus it doesn't matter if it's 10-15 minutes or the next day. I don't see the appeal of an external disk (that can fail as well) when you have to copy things back and forth for optimal performance. I don't even bother with that on my laptop where they are both build in.
Downloading patches doesn't take hours btw, unless you play something else while downloading. Hopefully that will be improved next gen so it doesn't slow down to a crawl when downloading while gaming. Hence I usually end up doing something else when there's a mandatory patch and won't play that game until the next day.