The benefit of both spouses getting cards is more with the signup bonuses than with retaining them for longterm, generally. My wife and I have both done the BA Visa, SPG Amex, and Citi AA last year. However we probably won't both keep each card variety once a year is up (unless a great retention offer comes along).

This is where FF programs like BAEC, that allow household accounts, can be great, as it gives you more flexibility. SPG permits points transfers between accounts at the same residence, which is yet another reason that card is a great!

But I'd also agree in general, be unfaithful and take advantage of whatever card offers you, and your credit score, can!