Personally I consider learning battle magic more as rolling the dice, but more in a career sense. Long term battle magic goes one of two ways. You eventually explode, or you get good enough with it such that it's become more reliable for you, via traits. Or in other words, you hope you roll high and get traits before you roll low and explode. (Or you get scared and swear off magic or get killed by some gribbly or whatever).
This is what learning battle magic is to me. It's not just learning the spell, it's then using it enough times until you get traits where you can confidently throw it around like seen in the Supreme Patriarch duels. Starting down this long and dangerous road in the first place is the gamble. Considering the number of battle wizard versus graduated battle wizards a heavy one even taking into account other causes of death, though considering it probably takes decades to complete this road they're hardly dropping like flies from miscasts either.
Now, the argument that not having battle magic is also a gamble has merit. You can either take the risky road of learning battle magic and possibly exploding, or you can take the risky road of not learning battle magic and dying when battle magic could have saved you because you live in Warhammer. These two roads are each dangerous and each have value, and in most people I think are a personal decision for the wizard in question to make.
The thing is, Mathilde is not other wizards and has a third road available for her. She already has what's effectively a battle magic trait in her staff. Taking this into account, I don't see any reason why on earth you would want to start at the beginning of that dangerous race to learn battle magic the normal way, ie learning battle magic you don't have traits and bonuses for, when she can learn mist battle magic in a far safer way already.
Said another way. You can either 1) not race and maybe die, 2) race and die if you lose, or 3) race with a shortcut and die if you lose. Whatever the usual arguments for 1 and 2, the existence of option 3 makes option 2 absurd, in Mathilde's unique case.
We don't (currently) have traits to help Rider battle magic, so I want it as safe as possible. If that changes I would love a trio or lance charge variant, but that's not currently the case. Maybe the Mist apparition will make a good battle magic if we can get the staff to apply? I would be eager to try that now, as AP hell allows.