Why is ESXi free? Is it a promotional thing, or is it a strategy of VMware?

I don't pay much attention to virtualization software, but bought Workstation when I needed to do some development on a couple flavors of Linux and wanted to use my existing work laptop. Eventually I got a dedicated dev machine, and ran Linux guests on a Windows host.

A quick search showed ESXi is a bare-metal hypervisor. I know there is a lot of competition in the market, but enough to give it away? I don't get any joy running my VM's on Windows on the dev machine, so is there any reason I shouldn't abandon Workstation for a free ESXi (I'm assuming I could still NAT my network?).