Modulability. As my nMP has gone on the fritz due to a faulty I/O controller chip (mainly having to do with Thunderbolt, ironically), all I had to do was plug 6 cables into my oMP to get my set up back up (plus a data migration via Time Machine). Couldn't have been more simple or easy. I'm not being sarcastic here either. Both machines are awesome for their own reasons, I'm just surprised how easy it was to swap from one system to another without loosing a beat. Maybe the external expandability does have a point.