The split process model is smart, since it ensures your shell wont crash if an explorer.exe browsing your filesystem crashes (usually because of a third-party plugin).
I've been running windows like that for some time and disabled it last week to check out what XPMyths has to say about it: http://mywebpages.comcast.net/SupportCD/XPMyths.html#Optimization (ctrl-f "launch folders in a separate process" will take you there).
I can report that I get a lot more crashes without it than I did with it, so am going to re-enable it now





- carpe noctem
, xplorer^2 being a goooood alternative), you won't be launching the second explorer.exe instance very often anyway.

Thanks for clearing that up