We do them "on schedule" for a few reasons.
1. We have no copay or deductible for well baby checks, so it doesn't cost me anything except the time effort and energy to take the kids.
2. I like having a relationship with the dr so he and I know each other, so when there is an actual issue, we talk easily with each other. I respect him, he respects me. But that respect has been built slowly.
3. I like to have a record of my child when he is well, so if he is sick, I have a baseline to compare it to. For example, my middle son had rotavirus and lost a significant amount of weight very quickly. My ped was able to look up his last several weights and realize that he had lost over 10% body weight, and knew to act more decidedly in treating him. He also knew that my son being lethargic was VERY out of character for him, he had seen my son bouncing all over the office numerous times before. Without that personal knowledge, the dr may not have taken stock of my parental observations the same way he did his own.
4. There are some things that I do not know, and despite lots and lots of internet research, would not occur to me to research, so I would not note as a problem.
A few examples include:
A. Cardiac conditions. Not all show outward signs that are obvious. I have a stethoscope, but I can't tell the difference between a murmur and a mitral valve prolapse.
B. A Pediatrician caught my "lazy eye" very early and treatment was able to get started at only 6 months old. My parents had not noticed it yet. They say that it became much more obvious shortly after, so they would have taken me to the dr for it, but sometimes delays in identifying problems can be bad.
C. A Pediatrician caught my sister's joint hyperflexibility in her foot and my mom was able to get her into physical therapy. Most likely my mom would not have noticed it until my sister was delayed in standing or walking, but the ped checked for it at the 4 and 6 month well baby checks and made a referral.
D. A Pediatrician pointed out my older son's sacral dimple and ordered the appropriate tests for it. I have a sacral dimple, and had not thought it unusual, so I would not have done anything about it on my own.
Those are just a few.
Now, there are plenty of negatives to well baby checks. Yes, it is an opportunity to push vaccinations. You have to be certain of your stance on issues like that before going to the dr. There's a chance that a bad dr could retrace a foreskin, give bad breastfeeding advice, unwelcome parenting advice, etc. You should try to choose your ped carefully so as to avoid those issues, or be upfront by saying "I have done my research and have chosen this method" and make it clear that your choices are not up for discussion (cosleeping, extended/exclusive breastfeeding, whatever). There's a chance that your kid could get exposed to germs at the dr, but I don't worry about that since my kids are out in public all the time and germs just don't scare me.
So yes, there are some potential negatives. But I think that if you are secure enough in your parenting decisions, and you have a reasonable dr, then well baby visits can be helpful in many ways. At the very least, I don't believe that they're harmful, except in the examples I gave above.