I'm ambivalent. I can and do live without these features. On the other hand, I'd make use of them if they existed.
I personally do eat all my fullness in one sitting, as you suggest most players do. There are two caveats about this, though, that I'd love the opportunity to verify at the end of an adventuring day. The first is that I actually did remember to eat -- in softcore I can pretty much look at my turn count and see that I'm 90 adventures further from clearing ronin than I'd expect to be, but in hardcore there is no similar clear marker and there is also no regular feeding time. The other thing that I'd want to verify is that I didn't accidentally eat to 15 fullness when I had Stomach of Steel.
As far as a spleen counter goes, I can see the possibility that I'd want to save a few spooky wads for when I was adventuring in the HitS, which means that I'd want to be sure that when I was ready to use them I hadn't blown my spleen on medicinal herbs. A spleen counter would give me the capability to see how far I had gotten toward that limit (and, of course, I couldn't write my own one of those since I can't catch the decisions that the auto-healer makes once I set it on its path).