Tbh this seems like a common-sense feature. Let the playerbase select which weapons they want as their primary or secondary weapon. It doesn't affect gameplay, just how often I have to press 2 whenever I'm playing Scout or Tank. If, for some reason, this can't be implemented on an account-by-account basis, then just look at the stats for primary v secondary usage and make the most-used weapon the primary. Our ring fingers will thank you.
(I hope these are the right people to tag. If they're not, sorry!)