Well I usually use the rules in covenants as a background element. As my current game is pre-1200, the ruling against breaing the economy by using terram vis to create silver does not exist, thus my players' covenant has a "silver" mine below it. This makes a moderate part of the pact's income (a third), thus solving the economic troubles the pact was due to be in otherwise.
We use Metacreator to do the accounting, so as long as income covers automatically calculated costs I don't care, ecomonics don't interest me anymore, we can thus concentrate on stories and not with book keeping... If I want an economic simulation I play a computer game or a board game, not an RPG (or I go back to work).
But a pair of such items would greatly break economics, thus creating major wealth in an area noted for its poverty (the alps) where huge agricultural production never existed, either breaking history completely or generating endless source of friction with neighbouring small petty nobles, maybe driving stories, but altering the focus of the game so far, in a direction not fitting my personal tastes.
So, yes it is technically right, but not fitting my game, if one of my players come up with the thought that this spell can be used to solve all the economic issues or food shortage in the area, I would certainly make sure someone would tae notice and steal it multiple times if required ! That might be fun though Or they might be smart and not overtly use it too much, thus doubling the food production of the pact, spreading rumors that their farmers are the best. Might work or not, depends if the beta storyguides want to follow this track with stories .
On the other hand, they have plenty of Vis, so they can afford it. I might talk about it with the beta storyguides.
Not directly related to the nourishment of temporary food ,
though it seems transformed people can be nourished in relation to size. Magi of Hermes , page 10 & page 13
Retreat as Flying Vermin
Mu(Co)An 35
R: Voice , D: Sun , T: Circle
This spell allows Alexander to turn a group of people ,
and their Animal-based equipment , into bats.
Base 20 , +01 Touch , +02 Sun.
Not sure, but I would expect the idea is that air is necessary but not nourishing. As long as you have air, you're OK. If that air ceases to exist, you better have some other air around. But if it's not nurturing, then it doesn't really matter if the air you survived on for the last ten minutes was magical. You survived and now you're breathing real air.
I don't know how air was viewed way back when in the biological sense. We don't want to conflate our current understanding of how oxygen is used in the body with the idea of air used in Ars Magica.
In an old thread someone mentioned that you needed air to refrigerate your body. No air, and you cannot expel heat, so you are toast. Literally. More or less it was something like that. Destroying air does not allow you to use that refrigeration conduit. Lungs of the fish does not prevent you from breathing air, but transforms water into air, so you get the full refrigeration effect. The fact that the air is magical is irrelevant since it is only a coolant, not a nurturing effect.
Air is necessary to convert the natural faculty into the vital faculty, which is what sustains life. It is not the air itself which does this, it just permits the change. This conversion is done with every heartbeat, so while the heart is beating, the vital faculty is made. Burns caused by magically created fire do not disappear with the fire, so the work done by magically-created air doesn't disappear either.
Food is necessariy to generate the natural faculty in the first place. If the food is not real, but lacks some essential essense/substance/energy/whatever, then the natural faculty created will be faulty and be of no use to the body.
It's like pottery - you convert clay (natural faculty) into porcelain (vital faculty) by heating it (adding air). Your clay is made up of earth and water (food). If your clay is not actually clay, but flour paste (magically created food), then you can never make a cup out of it (i.e. nourish the body).
How about that for a strained metaphor...
(yes, this is a post hoc rationalisation of the RAW)
I have occassionally thought that if we were to do it again, unlike the rules in Covenants in City and Gild or Covenants, we should do for medieval economics what Art and Academe did for medieval physics.
So, saving money is wrong, causes poverty and attracts demons. If you are a good ruler, your crops are nicer, as a mechanichal consequence of your benevolence, that sort of thing.
It does kind of wreck covenant finances and merchant houses as minigames, though, so I stand by us going the other weay this time around.
"I want the covenant to save money by cutting spending."
"OK, so you attract demons and your crops fail."
"WTF?"
So that's where all the 3rd ed demons came from! Any other fittingly strange economic beliefs?
I have a weak spot for the old black&white William O'Connor illustrations, they really catch moments of tension and drama. And isn't wocstudios.com/ravens.htm a nice illustration for the city of birds in RoP:M ?
I was thinking of something along exactly the same lines. To me, it solves many issues regarding what happens with the creations of some spells vs. others.
If you want to eat magically produced food to survive, go right ahead. You just warp faster.
Or they might be smart and not overtly use it too much, thus doubling the food production of the pact, spreading rumors that their farmers are the best. Might work or not, depends if the beta storyguides want to follow this track with stories 
.
