Workstation is an absolute must if you're running a lot of virtual machines.
1) The cloning option will save you a ton of SSD space if you create linked clones. The saved SSD space could pretty much be subtracted from program cost, since SSDs are still pretty expensive. And creating a linked clone only takes like 15 seconds, as opposed to whatever time it takes to setup an entire OS.
2) The tabbed interface is great. Instead of a bunch of windows loaded, it keeps them very well organized inside of a single window.
3) The live windows are also great for monitoring many VMs at once. It's kind of like aero sneak peek, except all the VMs show live snapshots (and update about once a second) and can be made as large as 640x480 I think. And this doesn't seem to tax my system at all.
4) Remote connections. I don't think this can be done in player, but you can hook up to your VMs from pretty much anywhere. I guess if you were to offer some kind of VPS hosting, this could be of use.
For me, the $120 I spent on Workstation 9.0 (student discount for the win) was so very worth it. But if you're only running like 3-4 at most, then Player gets the job done.