Working on one's offhand accuracy is a great thing, ask anybody shooting High Power Service Rifle matches, that's usually where the scores drop a bit after the prone and sitting stages.
Practically, I haven't had to use the offhand position save for those particular stages of competition and a small percentage of my hunting trips. One of the last Wisconsin whitetail I took utilized a tree branch as a supporting rest from the standing position, but for the most part I've taken them from the kneeling position, or sitting.
One thing I noticed when I joined the Air Force almost 15 years ago, is that we don't qualify from a standing offhand position with the M16. Tactically sound reasons, of course, but it's done from a standing, behind barricade, or prone, or kneeling, behind barricade. GI's tend to present big targets when standing up aiming at things. I'll bet this particular mindset carries over into a lot of other folks' shooting techniques, save for handguns.
There's not a darned thing wrong with honing one's offhand skills. If you can be a deadeye standing offhand, then you're on top of the game, and sitting, kneeling, prone, of off the bench should be an absolute piece of cake.
I do shoot my No5Mk1 Jungle Carbine offhand most of the time, but that's because the recoil makes bench shooting a bit sharp. Offhand rolls a lot easier for me.