Hi Bob,
Using a scanner and a UE for drive tests in a 3G network it will help you to spot missing neighbours more easy.
For example if you look on Ec/I0 plot for UE and Ec/I0 for Scanner and you see in some spots that there is a big difference you could say that there is a delayed handover or a missing neighbour.
For 2G I dont have experience.
I hope it helps!