That's just begging the question. The whole point here is that we shouldn't need "science" to somehow "justify" the colonization of Mars. Saying that it's a good thing because it "justifies" expenditure, is like saying that it was good for Hitler to continue WW2 because that's what incentivised the US to invade Europe: pragmatically it's correct, but pragmatism is really not what's debated here.
Personally I would like to see the priorities of "colonization vs science" reversed.
The expenditure does not need to be justified to people like you or me; it is obvious to us that we should colonize Mars even if it is currently a dead sterile rock. Sadly however I have not yet become emperor (work in progress), and funding for space exploration often requires short-term tangible justifications. This becomes more true the more expensive it gets.
From a pragmatic standpoint, it is important that any life that may be on Mars is discovered so that we can use those results to push for more funding. Anything that could accelerate the process should be investigated.
If that is unsatisfactory for you, I'll point out again that it isn't just a matter of politics and funding. The discovery and study of non-Earth life would provide countless opportunities for the advance of biology. An advanced understanding of biology is essential to the core mission of colonizing and terraforming Mars, even if I am made emperor.
I agree, but what you are saying is "assuming the status-quo, let's see if we can find something good about it".
Whereas I'm saying: you might be right, but the status quo is still wrong, and we shouldn't be happy about it being wrong.
Concerning your last paragraph - while it may be true, I'm pretty sure that gaining terraforming-related knowledge plays a very minor role in NASA's research decisions, which is also something we shouldn't be happy about.
Anything that we learn of biology will help with the eventual terraforming. That is not the sort of thing that necessarily needs to be researched directly right now, researching things in general expands our knowledge base, giving something for terraforming research to stand and build on.
Personally I would like to see the priorities of "colonization vs science" reversed.