As a house rule, I am considering ignoring the rule that says that future increases to Brawn/Willpower do not increase Wound/Strain Threshold. I don't see any good reason for the rule as written, and I prefer that there not be a 'trap' where it's mechanically better to start with Brawn/Willpower high and raise other characteristics than to start with others high and later raise Brawn/Willpower.
So, is there something I'm missing? Is there any good reason not to base WT/ST on current (Brawn + 2 * Toughened)/(Willpower + Grit)?