In what situation is the Shell Bell a good choice for a Pokemon? It seems like it would be a great choice for a more offense oriented Pokemon, but I rarely ever see it in a suggested moveset on the questions here on the site.
No, not really. Most Pokemon that are going to be attacking and getting Shell Bell recovery every turn would much prefer a different item to boost their damage output, speed, ect. Defensive/Support Pokemon will not be attacking every turn, so Leftovers is preferable for them.
First, the definition of a Shell Bell: "Whenever the Pokemon holding it deals damage, it restores 1/8 of the damage dealt as HP" -Bulbapedia
A good situation is when your Pokemon deals a lot of damage to the opposing Pokemon. That way, you will get a decent amount of HP restored. Also, if a Pokemon that is holding it uses Giga Drain or some other HP stealing move, you will get that HP with the HP gained with a Shell Bell. Other than that, it doesn't have much use.