VBR mp3's are very good, but it isn't FLAC. You don't need superhuman hearing to hear the difference, especially very dynamic music sounds better in FLAC. Hearing the difference becomes easier the better your audio equipment is.
There is another reason to keep your master music source in lossless format. Future recoding. Mp3 are excellent for every day use. I honestly can't tell the difference between high quality mp3's and the original sources.
What people don't realize is that mp3's are on the way out. That is a close to 30 year old format. AAC is the rising star but like mp3 it is a lossy format. So what happens when mp3 is no longer supported? You recode them to the new format.
Recoding mp3 to aac really isn't that big a deal. I can't tell the difference between high quality mp3 to high quality aac recode. But what happens 4 or 5 generations down the road if you keep recoding with lossy formats? Your music sounds like shit eventually.
Flac allows you to keep a master backup in perfect condition to go back to with the recodes. And if your recoding for space on your master source that is bullshit. 3 TB harddrives are around a 150 bucks. That will store a life time of music even in flac format.