(I'm in Bali having my 6:30am coffee and working on an ancient and venerable hand-me-down iPad and I'm not sure the animated emoticon loaded)
Go For It is one of my lowest lipsticks, because it's basically the best purple for me in all these years of buying purple lipstick, but I don't ration it, since I do have a BU. Also, it's one of my older lipsticks, from the era when I didn't have too many, so it got used a lot just due to that. Ever Hip is another one in that category, but I didn't BU, so no big loss, since I have others now that are similar but better.
I do have a few I didn't BU when they came out, that turned out to be ones I wound up loving and using a lot, even more than ones I DID back up! Ones like Reel Sexy, Sheer Mandarin and Strong Woman, but I was able to find reasonable BUs after 6 months or a year when the tubes were starting to get low (and of course, now Reel Sexy-like colors are all over the place, so I have others I like just as much, such as Sushi Kiss, Sweet & Sour and Riot House). It just goes to show that sometimes, you might not even know which ones turn out to be the ones you use the most. I wasn't sure I was going to find a Strong Woman BU that didn't cost an arm and a leg, but I didn't stop using it - there's always something new that'll eventually come around and it's true I'd have wound up using something like Private Party instead.
Some of my other low tubes I hadn't BU'ed are Pet Me Please (which came back, lo and behold), Double Spin, VG Gaga1, Call My Bluff, Lush Amber, Toxic Tale - see a pattern here? Mostly the oldest ones I had, so they were used more, plus softer colors that I'd wear during the day and do lots of reapplying with. Some others I love and have gone through a lot of the tube have been made perm, like Mehr and Sushi Kiss.
It's been handy to look over my most used lipsticks, because now I know I go through strong reds and vampy colors more slowly, plus I have lots of bright pinks, corals and fuchsias, so I know what I seriously don't need to BU. Doesn't stop me from buying new stuff, but I don't spend extra on BUs, at least. If I ever lose one that didn't have a BU, I will move on (after a brief mourning period lol) but I'm super careful about my most loved lipsticks so I haven't misplaced a really important one.