It amazes me that people still go on about this. Personally, I thought Data's death was one of the very few things Nemesis actually did right. I have to wonder if people would continue to piss and moan this much about Spock's death if he didn't come back in Star Trek III.
I wouldn't be - Spock's death actually made some sense in the story, unlike Data's, was brilliantly done for just the right emotional impact, didn't have a cheap get-out like B4 to essentially undo it within 5 minutes, and was a genuinely hard hitting but fitting
end to the character and the film. I loved Spock's death and funeral scenes, however hard they are to watch; in fact largely because
they're so hard to watch. I can watch Data's demise and not only feel no pang of sadness at all, but in fact often laugh at the mind blowing stupidity of it. If Data had died a death with the impact and poignancy of Spock's, it may well have saved that film.