I don't think many actually do the math on the diamond plan. With a family it makes total sense. Deductible is $4500 but the company kicks in $3000 towards it so you are only on the hook for $1500 then the plan pays 90% after the deductible is met. You are in the same network and use the same doctors as the part a and b plan. You save over $3600/yr in premiums. If you don't use all of the company's $3k it rolls to the next year and then you get another $3k so if you rolled over more than $1500 you have covered your entire deductible the second year and anything over that will start paying on those 10% bills should you get through your deductible and be where the plan is paying 90%.
Due to max yearly out of pocket limits you end up about even with the a and b plan in a catastrophic year. If you have a normal year the premium savings and the fact that the company's money rolls over you are coming out way ahead.
Caveat: if you take regular prescriptions the diamond plan is not good and certain prescriptions can be quite expensive.