I used to do a "monthly maintenance" routine with Onyx, etc I had tech tool pro, disk warrior, I checked all my disc every month, and then a few people on this or another Mac forum said stop wasting your time and just deal with issues as they come. That was about 3-4 years ago, and I have had no issues since. I tend to agree then that the maintenance routine is a thing of the mid 2000's when Macs were very unstable. I'm not sure the car analogy works, but I do take my car it in every year and a half for a/b services.... but not every month.
So it's not even about the money for the apps, it's more about the time drain, and that it may not be needed or even helping.
