when I picked up my first flatbed scanner with the ability to scan negatives (ScanJet 3570c) I scanned all my negatives. (2001 or so)
Later on in 2006 when the 3570 got toasted by a power surge, I picked up the HPg4050. I really liked the quality and I re-scanned some of my favorite shots and was very happy.
Now that the G4050 is old, and I learned more about scanning. I picked up the Plustek 8200i and then re-scanned a couple of my more favorite shots. The difference in quality from 2001 to last month is incredible.
While the 8200i cannot scan 120 or larger, the old g4050 does scan 120 and 4x5 pretty nice. it was terrible with color 35mm. I hope that Plustek makes a new 120 scanner someday.
I'm glad I didn't trash my negatives.
I've held off scanning most of my old negs, as it takes MUCH time! I couldn't imagine re-scanning and converting thousands of images several times, so I'm happy enough with the G4050 results I got a decade ago as useful "contact sheets." The photos I really like are worthwhile redoing years later with better technology.
I had an HP Photosmart film scanner in 1995 that was painfully slow and often produced streaks because dust would easily be caught inside. I'd spend 20 minutes waiting for a strip of 5 negs to go through, only to find out I'd have to clean the unit and start over.
I'd say that it's now safe to photograph negatives with a 36MP+ camera and macro lens and be able to get all the detail and dynamic range you want from a 35mm original to make quality enlargements that rival silver emulsions. In that respect, I can accept the arguement that it's okay to destroy some negatives. Just as scanning technology has improved, so should post-processing a good RAW capture. At the moment, we've pretty much reached the limits to MP and lens resolutions for full-frame sensors, which was the original problem with 35mm film also! Each silver grain has multiple and overlapping pixels of data (for most films). Is it "good" or just "good enough"? Only you can decide.