I think the Rogers Waters and Pink Floyd situation is complicated. He definitely made an important contribution artistically with his lyrics and writing the music to many key songs. I also agree that Gilmour and Wright contributed hugely to the music writing and overall arrangements and sound. I agree Mason contributed the least from an artistic perspective, but that said, his relatively plodding and unimaginative playing is very much part of the sound. When you hear Floyd covered by others (including Roger Waters), it can sometimes sound "wrong" in the hands of a more skilled drummer.
I don't think it is correct to say that a bassist can't contribute as much to a band's sound. People like John Entwistle, Chris Squire, Geddy Lee were all essential components to their respective bands' sounds.
In terms of the who-treated-who badly aspect, it is impossible to know what really went down. Some people say Waters treated the rest poorly during the latter years of the band. Waters says he's been treated badly. I'm willing to believe there was nastiness both ways and also willing to believe that perhaps more than 50% comes from Waters. Although I haven't followed this issue closely, it seems that Waters was most angry in the earlier years after The Final Cut and then has mellowed more recently. This happens. Lennon was quite sharp and critical of McCartney and The Beatles in general right after they broke up, and then became more balanced about it over time.
One small correction to the above, Welcome to the Machine is sung by Gilmour, not Waters.