It's not a matter of opinion. It's just fact, plain and simple.
In the old days (ha!) people left water out to evaporate chlorine and didn't need conditioner.
These days, most water companies use chloramine
instead, which bonds to the water harder and takes 2 weeks to evaporate! Great for people, bad for fish.
There's also heavy metals to consider.
Water conditioner costs a couple of bucks, jeez... less than a burger and fries, to make sure your kid's pet is healthy and doesn't drop dead on her...