Why does everyone forget the line...
You can't use a symbol of death offensively
Once triggered, the symbol becomes active and glows, lasting for 10 minutes per caster level or until it has affected 150 hit points' worth of creatures, whichever comes first
Symbol of death can be made permanent with a permanency spell. A permanent symbol of death that is disabled or has affected its maximum number of hit points becomes inactive for 10 minutes, but then can be triggered again as normal.
I would say that the permanent symbol lasts for 10 mins per caster level or until its max HP have been affected (which ever comes first) then it shuts down for 10 minutes and resets.
Actually, there's an example that suggests the shield trick works. From Baba Yaga's stat block:
"Baba Yaga has permanent symbols of death (DC 36) magically inscribed on the center of her chest, between her shoulder blades, upon the crown of her head, and on the soles of her feet. Only a few great beings have knowledge of the symbols’ passwords (each symbol has its own unique phrase), and even fewer significant personalities have been attuned to the symbols."
I doubt Dear Grandmother is worried about peeping toms. This is clearly meant for use in a fight.
There's no RAW label of what is offensive and defensive and it isn't a matter of common sense, it's highly subjective. My best defense against an assaulting bandit may be making it painful or even deadly to glance at my shiny robe, after all the best defense is a good offense. So far I've seen one example of an official Paizo stat block of a Symbol of Death being used in a similar manner and no examples to indicate it isn't legal to use it in such a way. If you can find some examples for me to prove otherwise I'll take your word, but otherwise please don't be condescending by insinuating I lack common sense for a perfectly reasonable conception (or possibly misconception) of the rules.