Evolver, the marmite is probably popular due to the high salt content.
I read somewhere that cats can't digest fruit, so I guess a little bit shouldn't harm them, as long as they were getting everything else they needed in their diets. Animals such as dogs definitely need meat in their diet (I think cats as well). The family of an ex of mine were vegetarians and decided to do the same for their dog. They fed it a diet of wheatgrass juice. It died a week later.
So as long as the cat is getting is other stuff, then the fruit shouldn't harm him, but would I know I'm not a vet. To spice this thread up I've included a link to the world's fattest cat
[boreme.com]. It's really quite impressive and worth a look.