I had one at 41w to check fluid levels, but I didn't get to see the screen I was pretty over it by then anyway so I really didn't care!
A friend in my MG has been having them fortnightly as her daughter has a lung condition (kinda a tumour) that they are having to monitor the size of.
If there is nothing wrong, I think all scans in 3rd tri are optional and therefor you dish up the dough! But Chrissy - if youa re keen for a 4D scan, there is nothing wrong with you requesting and paying for that scan!