I don't know at what temp oil starts to get damaged, so I can't say whether or not a cooler is really needed, but I can say it really reduces oil temps. When I built my bike I got an oil temp gauge from Europe (they don't sell them here). Below is a pic of it after a long steep technical singletrack climb (the long end of the needle is the one you read). Every time I got on singletrack the needle was in the red. There were probably a dozen times that summer when it went clear off the top like pictured below.
Shortly after that ride the clutch started slipping and when I replaced it, the center disks were clearly burnt. I found and installed the Jagg shortly after that and now when I'm on the really nasty stuff I'm barely above redline, but nothing near where it was before.
I hadn't seen that page on your website before. Thanks for pointing it out. Just curious, do you know how much DR. Z sold his oil cooler for?