Agree 100% with alefor: a wifi hotspot is always more useful than a USB modem, in my opinion. Of course, my hubby and I are always traveling together, so we like that we can connect multiple devices. But even when you're out for coffee with a friend or something, it's very handy to have your own hotspot and connect any device you need to it.

As for convincing the wife (I have a similar, but different, problem! ) that it's necessary: will you be more easily reachable by her? Will you win money on bar trivia? Do you have a website or email account you need to check frequently for work that brings in extra $$?

If that fails, I totally suggest getting HER a wifi device so that she'll have something to use when you're both out. (It pays to be sneaky.)