If your travels take you to the "Hilton" part of the Hilton Family (normally in urban areas), then Diamond is worth it with Towers Level probabilities.
If you are going to be using Hampton Inns or Homewood Suites, which GENERALLY seem to be in suburban/rural areas, there is no real advantage in my experience.
As Doc noted, however, if you're close ...go for it...as you never can tell when you might need the assured reservation feature.