I do feel as if Brosnan is unfairly dumped on by certain Bond die-hards. The big issue as I see it is that Daniel Craig has re-invented Bond like no other previous actor has and like with anything in that sort of situation, it's easiest to compare the new breed (Craig) of success with the most recent form of "traditional (Brosnan)." When Dalton took over for Moore, yeah the tone of the films changed slightly, but it was essentially the same formula, it wasn't a drastic change of direction so people were mainly only comparing actor to actor at the time. Brosnan has to contend with not only the actor, but the completely different tone, style of Craig's films.
I have to disagree with you a bit here. I'm one of those prone to dump on the guy and my opinion of him was formed entirely from comparing him to previous actors. I'm old enough to be of the Moore generation (TSWLM was the first I saw in a theater), so I remember when Dalton took over in the role. I think you're right that it's easy to overstate how much the tone of the films changed from Moore to Dalton, but nevertheless Dalton's portrayal and the movies he was in were
of quite a different style from Moore's. Dalton really did bring something new and interesting to the role and the more down-to-earth nature of the plots was very refreshing after some of the rampant silliness of the Moore era.
I liked Dalton a lot so I was definitely disappointed when he was replaced. I thought GE was OK, but it was also kind of a let down for me. It was clear from that film that EON was discarding the more serious take that they'd tried with Dalton and going back to formula. Even before DAD I was convinced that Brosnan was going to see the franchise burn out because you can only remake the same movie over and over again so often before people realize you've got nothing new to say and then turn to something new. (After DAD I prayed
that the franchise would burn out, but I was already starting to become indifferent about seeing a new Bond movie before that well-known travesty was inflicted upon us.)
I guess what I'm trying to get at is that the fact that Brosnan gets dumped on doesn't necessarily have much to do with Craig. I was actively dumping on the guy 10 years ago when I had never even heard of Daniel Craig. In fact it's probably easy to make too much of how the tone's changed. It was really impossible for them to have kept turning out the same old stuff movie after movie like they did with Brosnan and still have people remain interested in watching. They had to try something new, so the movies would have had to change quite a bit no matter who took over from Brosnan.