We have too little information on Mars to go in at this point and try and reconstruct it to be "Earthlike," even if it was feasible.

At this point Mars is a science study, and everything should be done that is possible to try to avoid adding variables until we know where we stand. Is there life? Was there life? Can Earth bacteria infest the planet? I could list questions all day long like this.

It's like a construction site that hits an ancient archeological site. There may be arguments about who should pay for it, but ideally someone pays the construction company for lost time, etc. and the proper scientists remove the artifacts and catalog what needs to be cataloged. Eventually, barring further problems, an all-clear is given and the building can go up. Scientists happy, builders happy.

We need to look at Mars for many, many more years before we screw with the whole planet in some permanent manner. If at all. Personally, I don't think we need to terraform it anyway. I think a lot more could be done building O'Neill colonies and such out of asteroid materials, and the technology would be a lot more achievable. Terraforming Mars is not a critical stepping stone for human survival, or even the exploitation of space resources (asteroids are much more convenient. Gravity well and all.)

Needless to say, manned science stations and mining and other operations to support such stations would not be a problem, since they are unlikely to affect the Martian environment other than in tightly localized areas.

If it turns out Mars is hospitable to Earth microorganisms in any way, i.e. they can reproduce and spread, it's going to make things a little tricky.