All seem to be in agreement that 2.9 cents a mile is too much. During the double miles promo - 1.5 might be worth it.

The real question is whether or not you believe 1.5 cents is a reasonable price. Not whether you should pay your mortgage this way. As others point out, if you decide this is a good price, then don't stop at your mortgage. Pay that the usual way, and just transfer as much money as possible to your wife's account, effectively buying as many miles as your credit limit will allow.

I don't want to presume anything about your finances or credit limits, but if you have available cash in low-interest or no-interest bearing accounts, a better bet would be to buy US Savings bonds (do a search and you'll find a lot). There is no fee - and with double miles you could get a lot of miles for free and earn better interest than in current savings/money market accounts.