Adam said:
"If Mars-life has been reduced to a few oases I'm sure it would appreciate some improvements around the place..."

Lets imagine the situation from the other perspective, suppose there was intelligent animals on the planet Mars. (lets call them arfs) They are studying the Earth right now and say:

The "yipes!" part should be the damn Luddites who believe that terraformingEarth would be "mucking it up" - as though a Oxygen-laced, nuclear radiation-blasted planet couldn't be improved upon! AFAI am concerned any Eart-bugs will either be our ancestors or our relatives, since Mars/Earth share essentially the same biosphere. If Earth-life has been reduced to a point where the animals must kill each other in order to survive, I'm sure they would appreciate some improvements around the place...

Would you object to their "improvements" to our planet?

Personally, I would say that terraforming would be OK "IF" (and that is a BIG if) the planet is devoid of life.
Better to find out what the "life" there is all about before changing their habitat.

Johannes