The point is, that they decrease the invasion chance a lot. Without them you would have probably around 30%, if you had 1%. Also the 1% is the Base Chance. The more noise you make, the higher the chance gets.I thought the Imperial Guards and the Prison Guards were different beings. Since you start with the Imperial guards but it brings up how if you switch to prison guards to cut the cost at the cost of the chance of an evasion in your office.
Either way, the Imperial Guards are useless. 1% chance and got invaded. What is even the point?