Development. Particularly spinning up multiple VMs to test multi-tier applications. Need to ensure load balancing works, database failover works. Testing against databases with large working sets for a given query (analytics and DW, not OLTP). Spinning up virtualized Linux clusters for doing development against Vertica.
Small websites, iOS development? I don't need 16GB for that. For work stuff? I absolutely benefit. The Thunderbolt MacBook Pro I bring to client sites can devour its 8GB rather quickly. When 8GB DIMMs get cheaper, I'll probably bump up to 24GB.
Aside from that... I've had rendering jobs back in the day devour memory at a pretty ferocious rate. I did a large Minecraft map render recently that would've been sped up if I wasn't paging to disk as often (thing was clearly unoptimized, as the total commit charge for the render was 52GB).