Well, they do when it's followed by "we shouldn't keep all our eggs in one basket". The only possible (though still extremely flimsy) reason for creating a Mars colony is if we think we can eventually make it self-sustaining. A non-self-sustaining colony on Mars is worse than useless.
sure - however a non-self-sustaining colony on Mars might be the first hundred years, a self-sustaining colony may be the end goal but end goals may be long out.
It is very debatable whether a current-tech Mars colony being sustained for a hundred years is in any way a pre-requisite for a self-sustaining Mars colony.
I would claim not - at least some of the biggest problems that we need to solve to create a self-sustaining colony in a harsh environment don't actually need us to be on Mars - there are significant problems we can first address right here on Earth. This is significant because unless and until we can solve these problems, there is no reason to try to establish a base on Mars.
