Count in other grinding games that are on facebook (Farmville, etc.) and you will see that the number of players that are entertained by grinding is actually a substantial part of the playing community as a whole. It doesn't matter whether the game is "hardcore" or not.
Seeing the rather extreme case that these many years after release, there are still that many people playing Borderlands (and Diablo) just for the grind is an interesting thing.
If you could get your sims characters to craft items with a probalistic quality curve based on their personality, skills, and some luck, and also have an internet-facing display chest, it'd probably give even more legs to the franchise.
See also Animal Crossing, which gives people plenty of collecting and display opportunities. There are fossils, furniture, fruit, paintings, songs, haircuts, fish, bugs, etc etc. AC appeals to casual gamers because you can do 10 minutes a day and get enjoyment from it. It also appeals to hardcore collectors because there's so much to collect, and some of it is "rare".
If this list (http://en.wikipedia.org/wiki/List_of_best-selling_PC_video_g...) is accurate, for instance, Diablo and Diablo 2 sold about 6.5 million copies together, which ain't hay; but The Sims, Sims 2 and Sims 3 together sold 39 million copies, making it a completely different magnitude of hit. (Not even counting all the expansions, either.) And how many hundreds of millions of copies of Angry Birds have been sold?