What are your personal policies for selecting which nodes should have testpoints?
Can a testpoint be used as a node for powering a circuit?
Does a testpoint on a line create a stub, subject to interference or crosstalk, making it better to not have a testpoint in some situations?
Do you put testpoint on I2C and UART RX/TX lines for debugging?
ground should always be a test point, power supplies are good test points too, because if the power is bad that can cause unusual behaviours. other signals that pass between circuit blocks make good test points too.
some circuits can be powered by the test points
stubs, yes at several gigahertz, not usually a problem except on fast circuits like PCIe
yes, especially if those lines do not emerge on external connectors.