Hi Jaimee,
I would try to press the doctors much stronger on defining what is "necessary". In my opinion any test that would indicate a health condition on YOUR part that could endanger the child is necessary. So if they do a glucose tolerance test, or anything like that, I would consider it necessary. However, if you are not interested in knowing what the odds are of your child having any particular kind of chromosomal abnormality, then those tests are not necessary.
I suspect that doctors want to say this is necessary for two reasons. One, they might get a kickback from the lab that performs the tests; two: they like to contribute to medical knowledge through medical statistics.
They indeed can tell a lot from the ultrasounds (which are of course wonderful), so just don't get the other tests done, if you don't want to. However, if cost and worrying are not an issue, then please know that all the tests done in the first round (ie if the tests don't indicate something) simply involve drawing blood. Nothing dangerous to the baby, just to your sleeping well at night until you get the results.