Neighbor and MF Lists are static not dynamic. You may like to add 1-2 backup neighbors in such scenarios but normally we optimize the network for normal situations. I mean if defining unnecessary neighbors is harming the performance in normal hours which it really does then you wont define them just for backup. Because sites don’t go down so often and for long times normally.
the more neighbors you define, the harder it gets to do the frequency planning. two neighbors cell shouldn’t use co-channels or adj. channels, so it puts a lot of constraints to have too many neighbors.
Second thing : the MS takes more time to listen to all the neighbors, therefore the ho candidate selection is less reactive at BSC side.