I work in a vet clinic and people do this to the doctors as well and its funny to see them complain about how they only went to school for how many years and then this person asks a whole bunch of questions and then does the opposite of what they suggest. Why even bother asking a vet what they think you should do if ultimately you know that you are not willing to do what they say?
I generally feel the same way as the OP but there are exceptions...especially when you make considerations and adequately address concerns that are brought up.
As for the Vet example...I did this to a vet with my Fennec Fox because I had several reports of incidents where vets were giving fennecs full dose vaccines and killing them (there are no approved vacs for fennecs). So even after that talk, he I think started to give her the full dose and I had stop him and insist on half doses, and I'd come back more often for boosters if needed... I basically put it to him as, is there harm in doing half doses? Should I ignore these reports of fennecs dying to full dose shots? ...He had no counter arguments, and was like ya I guess its best....so there are exceptions, especially when dealing with an intelligent person who has done some research.
Another example is I am almost certain I had meningitis...I had most of the classic symptoms (stiff neck for over a month etc..), multiple risk factors such as being exposed to bacteria and fungi in vivariums and tropical plants/animals, my place was a mess, I had mice which can carry a virus that can cause meningitis and I think a couple of prescription meds I was can cause it or contribute to it.... So basically I told the nurse all this and she blew me off...said I'd be in the hospital if I had it...ya if it was acute, but there is regular meningitis and acute meningitis...Luckily I didn't have acute or I probably would have been dead...but as I was broke I went to the pet store bought the suggested antibiotics (for fish LOL...but researched that to and couldn't find much in the way of actual health incidents) to treat meningitis and basically cured myself of whatever was going on in about 2 weeks after suffering symptoms for over 2 months probably. Did I truly have meningitis? ...We'll never know, but I had the symptoms and the prescribed treatment for meningitis had almost immediate effect on my symptoms and then made me better (Only I took large oral doses instead of doing IV)
There are times when the so called "experts" like doctors/vets get lazy...or like a lot of people forget half they stuff they learned, never had experience with it (like the vet who was treating my fennec only vet in town who would and didn't have a rep for killing animals) and possibly are not keeping up with new information... So it is good to be informed, ask questions...and in some instances ignore advice or just take it into consideration and go do your own thing or get a 2nd opinion.
It depends who you are, and what the situation is....If you're semi retarded and can barely tie your shoes, probably better to just follow the DR's advice, but if you have an IQ pushing 140, can understand practically anything you read in any field and are very good at finding info/researching things...your own take on the situation may at least be valid....and should be considered too...just as you should at least consider what the "experts" say.