i always wondered and always forget ot check. there is a skeleton (champion i believe) that always summons another one. so if oyu summon it does it also summon his own summon?! If so then they are quite powerfull.
Summons cannot create secondary summons. Use the link below and you'll notice that the summons version of the skeleton champion does not have the Summon Skeleton spell.Link
well another thing that ghosts have is that they can not be damaged by normal weapons so guards and bandits should be prety scared of them.
Actually, most guards carry silver weapons even at low levels (it's how I got my silver bow at level 1 recently, thanks to the lovely unicorn) so guards will be able to fight off ghosts. Bandits are a different story. Additionally, summoned Ghosts
only have a 50% resistance to normal weapons while summoned Faded Wraiths
have a 150% normal weapon resistance and 200% resistance to both poison and frost, making them stronger than the wraiths you'd fight (well, in numbers anyway).