I don't necessarily agree with interrupting your cast for a firestarter proc, but I do agree with some of what you're saying, especially letting firestarter procs stack with each other.
An example is when you're casting fire 1, you get a proc at the end of it and you begin to cast another fire 1 ready to use up the proc. But then you get another firestarter proc at the end of that second fire 1. That there is a wasted firestarter proc, which by all means should've counted as a second proc. You've won the dice roll, you've got the proc, it will still take up a GCD, why shouldn't you be able to use that second firestarter proc?
Tbh, I think firestarter procs should really proc right at the beginning of your fire 1 cast anyway, the proc would disappear if you haven't finished casting fire 1 and you can only use it after casting fire 1. Therefore giving you a chance to react to that firestarter proc, instead of reacting to it during your next fire 1 cast.