Exactly. They can't colonise the planet so much as set up a self-sustaining enclosed dome on its surface or somesuch. (Which they certainly can do – they presumably had a self-sustaining ship, after all.) This doesn't provide a mass emigration destination, it doesn't provide humanity resources, and it doesn't even provide science experiments that we can't do in our own solar system. Now, I'm as sentimental about exploring the universe as anyone, but at any believable technology level* this seems like enormous trouble to stick some people on a distant world where they'll do little more than live out the rest of their lives.
I haven't read the Red Mars trilogy, but as it happens I have read a summary. And it agrees with what I said: terraforming takes generations. Also, quite tremendous resources. How much equipment are we expecting to haul there?
* It occurs to me that we will probably exceed what I consider "believable" within my lifetime. This may be my problem here.