That's not exactly a scalable solution. You have to keep in mind that large companies often have thousands of requests they have to process every _second_. VT is great, but I doubt they'd be happy if they were getting DDoS'd by every major corporation that wanted to run signature checks they didn't have cached.

EDIT: I also think you're misunderstanding. Symantec Endpoint Protection isn't the one that has multiple AV engines. Their (well, Blue Coat's) CAS appliance uses them.