A 21 GB single Runtime? Wow...

I have over 10 Runtimes already, though you have over twice the amount of hard drive space invested in content.

Anyway, IMO it's not a matter of space, but of organization. For instance, I am a big fan of Aery Soul products and thus I have a single specific Runtime with every item of theirs that I own. I also have a DAZ Gen 3 Runtime for Victoria 3, SP 3, Aiko 3, etc. I have a DAZ Gen 4 Runtime for, you guessed it, Victoria 4. I also have a Custom Runtime for assorted content from Renderosity, an Apollo Runtime, a GND Runtime, etc. And that's how I see it - I organize Runtimes to help me find my content faster. It's easier to just go into the Daz Gen 3 Runtime and pull out Vicky 3, then into the GND Runtime and morph her into GND2 rather than go through a mess of folders in a single Runtime to find the items to do that.