What are some convincing arguments to upgrade from Visual Studio 6?

Question

I have a client who is still using Visual Studio 6 for building production systems. They write multi-threaded systems that use STL and run on mutli-processor machines.

Answer 1

Though I no longer have concrete details, I'll just throw in that when we upgraded at work, the new compiler found quite a few errors that VC 6 let slip through quietly. Improved product robustness just from the upgrade.