I think it can be an unnecessary procedure. Any procedure, no matter how minor carries risks. If people were responsible pet owners it would largely not be needed.
Also, some vets find the reduction in aggression claims somewhat overstated if not dubious. It can prevent sexual hyperness and bad behavior in some dogs where that is an issue, but again the benefits are often overstated.
The only real tangible and pragmatic arguement for is it prevents animal shelters from being full of even more neglected animals and animals being put down because they have no where to go. Again, if people were responsible...
From an ethical / moral standpoint you could get into the concept of personhood and partial personhood (we ascribe a certain level of personhood to our pets). The arguement being: If the procedure would be unethical to do to a child why would it be ethical to do that same procedure to an animal?
The answer is we, society, do not assign full personhood to pets. And so the ethics of how we treat them is different.
Though this has changed rapidly over the last 50 years or so and the relative amount of personhood we ascribe to animals has increased significantly. I suspect it will continue to evolve.