Well, one thing I know is that as long as you're a partial member, whether you bank with them or not, you'll never get any offers under your "My offers" page, so I don't know if banking with them comes into consideration when you apply for a card.
DId you end up getting approved? Do you remember about what your credit score was at that time? Thank you by the way!
It is possible that having a usaa checking account that you run large sums of money through for years (thus creating a real history) might be a positive factor in getting credit from them (though more likely to influence manual underwriting than the computer ). But just having a checking account won't make any difference. At least according to the rep I spoke with in February when applying for the mc.