This is another in what I'm anticipating to be a series of "But why bother" threads (see also my questions on Automata). In this case, I'm trying to work out how and why Hermetic Theurgy works.
From page 78 of Mysteries revised we get the description of Hermetic Theurgy, the virtue allowing the commanding of spirits to do the spell-based bidding of the magus.
I just need to work out why it's worth its weight.
If I feel I might need a Pilum of Fire spell, I have a couple of options. I can learn the spell or invest it in a device or I can learn an "invocation" spell and let the spirit cast the spell for me.
So, let's assume I have Creo 15, Ignem 15, MT 5, Int 2, Stamina 2. I also know a Name of Power (Ignem) at lvl 20 (for a +4 bonus I think). Assume also a Penetration of 3.
Invent and cast the spell:
Lab Total: 15+15+5+2 = 37 + Name of Power (Ignem) 4 = 41. This is enough to invent the spell outright.
My casting total is 15+15+2+4 = 36. That gives me a Penetration of 16+3 = 19.
If I invent the Hermetic Theurgy spell (spending only a season, I grant you):
Lab Total: 15+15+5+2 = 37 + Name of Power (Ignem) 4 = 41. This is enough to invent a sufficient spell (lvl 20) with no additional penetration.
To gain the PoF effect, you spend a round casting the Invocation spell and then a round to instruct the spirit.
Put like that it just doesn't sound worth it. Granted, with a high enough lab total a high penetration version can be invented, but if you can do that, you could have faster access to the spell through putting it in a device.
So, what's the deal? Why get the virtue and why invent the spells to use it?