As well as the charges issue there are three other points.
They are delivery reports not read reports.
Because of the way they are implemented they are low priority on the network and will be dropped at busy times. (This means the lack of a delivery report doesn’t necessarily mean it wasn’t delivered)
They don’t work reliably across different message centres. If you and the recipient are on different message centres, You’ll get a delivery report when it reaches the next message centre. (This means that a delivery report doesn’t necessarily mean the message was delivered)