The entire thing was a plot device so they had some way of being able to single out Anakin as being "special". Also one of the problems with the idea of The Force being a simple mystical force that anyone could use with training and practice is that if that was all there was to it, you'd see tons more Force Users running around everywhere as opposed to a select handfull of people, and it would have been very difficult to wipe them out to near extinction.
Now granted, the whole thing was a bit dumb, but I understand why it happened, and why it had to be done. Also I'm not sure if it's really them gaining power from the symbiotes (they aren't parasites) or if they are a side effect of being favorted by The Force.
Understand that the entire thing is still pretty mystical, I mean we are dealing with a concept that relies on things like prophecies. The big "point" of 90% of what happens is that there is a prophecy about Anakin that he will "Bring Balance To The Force". It's believed by some that since The Sith are appearing again after having been entirely wiped out, that Anakin is supposed to put them down again. Sadly that's not the case, since the good guys are dominant he's supposed to bring them down, create a period where things are "balanced" and from there will come a new age of evil and strife ruled by The Sith, much like the age Papaltine refers to. A big part of the story is that just as the good guys are misinterpeting the prophecy, so is Papaltine. Papaltine believes that a new age of The Sith will come from this immediatly and he'll get to be in on the ground floor as it's leader. He's also wrong, he and Anakin are simply tools to create the transition period.
Incidently, that's why The Sith are so powerful in the prequels is because the universe is on their side. The good guys mention how "The Force Is Cloudy" because they are also becoming weaker due to what The Force itself wants.
The entire point of the KoTR II video game was about free will within this concept. The ending got messed up because they had to go with an earlier build, but the whole gimmick of the game was someone (Kreia) trying to destroy The Force itself, so that people could determine their own destiny. Albeit other than killing the force users, how she planned to deliver the final blow after that point was never revealed. Apparently George Lucas was involved in this storyline.
The point I'm making is that even with the whole "blood symbiotes" bit which annoyed people, I think the entire idea of "The Force" is still pretty bloody mystical. Given that there are prophecies and such (and these elements have been present ever since Luke visited The Cave on Degobah in "Empire Strikes Back") obviously there is an intelligence of sorts involved, whether that intelligence is some godlike figure in white flowing robes, some kind of symbiotic mass mind that lives in all things to some extent, or a prevasive energy field which said symbiotes help with the channeling of and are put into living beings by the chosen few... it all comes down to the same basic premise.
I guess the point of what I'm saying is that while the hatred is understandable, I don't think it actually changed all that much in a functional sense.