Couple thoughts:
Valuing diamond is tough....There are many different methods to use as can be seen throughout this thread.
I'll value it by the amount of mattress running I am willing to do should I fall short of the requirements. For me, the breakeven point there is about 10 stays.
Locally, that works out to about $1000 (at $100 per stay) less somewhere in the neighborhood of 15,000 Hyatt points. If I had a lot of stays planned for the subsequent year at some really nice Hyatts, I'd probably be willing to stretch that out further - the opposite if I did not.
And on the FHR/virtuoso rates, I agree that diamond is less valuable if you typically pay for your stay at the Hyatt Daily rate (which is usually pretty close to the FHR rate). The problem is that you cannot take advantage of any special rate (AAA, myelite, AARP, etc.) or best rate guarantee or stay on points and get any of those benefits. Personally, it's pretty rare that there is not some sort of advantageous rate I'm using when staying at a Hyatt. And I save my points to redeem at the nicest Hyatt properties.