Yes, You'd better have a back-up glasses even if you have contacts. Sometimes you may get eye problems, like infection or have a cold, and you cannot wear contacts, at this time a pair of eyeglasses is needed to help you see clear. If you go travelling, a back-up pair of glasses is a good option in case you need it.
Though a back-up pair of glasses is a good choice, you can still refuse to get them if you don't want to get them. As long as you can take good care of your contacts, there is no need to get another pair of glasses.