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.
If order-of-development effects like this are your primary concern, why not allow players to rebuild their characters as long as they end up with the same characteristics, talents and skills? So if a character who starts with a 3 in Brawn but a 4 in Agility uses Dedication to raise his Brawn to 4, he can pretend he started with Brawn 4 and Agility 3, then used Dedication to raise his Agility to 4. This has less chance of affecting the game's balance than a house rule (though I doubt your house rule will have much effect). It also allows optimizers to develop their characters organically instead of planning characters in advance so they can take advantage of talents that are cheaper in one specialization than another.
Wouldn't that be an unnecessarily convoluted way of accomplishing what this fairly simple house rule already achieves?
It's more complicated, but I don't think rebuilds will come up enough for that complication to cause problems. After all, Willpower and Brawn are the only stats that have a special effect at character creation, and unless the game goes on for a long time, most players will only reach Dedication a few times.
Having said that, I don't have any objection to your house rule. I merely wanted to offer an alternative that didn't have any chance of affecting the game balance, as that seemed to be your primary concern about the house rule.