I do like how normal casting is portrayed in this system. It seems like this accurately characterizes how Harry is generally completely spent after even short battles in the books,
at least until sponsored magic comes into play later.
However, it does seem rather limiting that even an expert caster could probably only get off at most, lets say eight spells (mental stress track + consequences and maybe some physical consequences for good measure). We have instances in the books where
wardens are described as having fought in pitched battles that last most of a day (at least, that is my current interpretation). When Morgan almost took down the Red King in battle are we to believe that he only used on the order of eight spells in the course of that battle?
It seems to me that there are a couple of reasonable solutions/interpretations to this issue:
1) Possibly RAW is sufficient and it's just standard operating procedure for wizards to fight in shifts (I would guess three) so as to trigger the rest between scenes rules to clear the stress track. Thus, whenever we see a group of wardens engaged in pitched battle in the book,
such as when Luccio and friends are protecting the kids from the zombie hordes,
they were ultimately screwed since they would quickly burn through their magic (i.e. mental stress),
until Sue showed up to save the day that is.
2) Another possibility is that we should add a couple of stunts that full battle mages would take. I'm seeing one stunt (battle mage or whatever) that allows you to cast magic with no stress if your discipline roll is double the shifts of power you decide to use. You could probably add another optional stunt chained off of the first stunt (call it battle master [originality for the win!]) where simply achieving spin on your discipline roll to control your shifts of power (i.e. power+3) obviates the need to take stress.
3) We could allow for a new mechanic where a pertinent loose (non sticky at least, I can't remember the terminology off the top of my head) aspect on a character can be tagged by that character to cast a spell and the use of that tag would be to cast the spell for 0 base stress. An example of this would be that a character could do a navel gazing conviction maneuver to place the aspect gathered power on him/herself. Then, next turn, could use the free tag of that spell to cast without taking mental stress.
4) Finally, it could be argued that Sponsored magic is much more common than I would have thought and that most mages expecting to enter battle would find one or more sponsors that they could incur debt with in exchange for powering their magic.
A somewhat related question I had was how you would portray the battle in Small Favor where
the aquarium was cut off in a giant circle which limited the ability of people in the circle to cast magic. Harry had to take a deep magical "breath" and store as much power as possible so that he would be able to use his magic. Mechanically, how would you describe that?
tl;dr Long post is long, what do y'all think about taking mental stress for casting pretty much every spell?