Time4Tea: Really, it's very similar to RPG side-quests. They shouldn't be essential for completing the game, but there should be some sort of reward for doing them.
RPG side quests that do need to be completed to feasibly beat the game (particularly if it's your first playthrough) can be done well, particularly if the game's point is being less linear.
For example, looking at Super Famicom RPGs made by Square:
* In Final Fantasy 6, once you get the second airship, the game becomes non-linear. While you can go into the final dungeon right away, it's intended that you do sidequests to pick up companions; doing so gets you extra treasure, and you also get the XP you need to make the final dungeon not too difficult. There's also story content in these sidequests (though it fortunately isn't as cutscene heavy as the game is prior to getting the second airship), and the game guides you to specific sidequests (like the ones that give you Cyan, followed by the Gau one) so you don't need to wander aimlessly.
* Final Fantasy 5 and Chrono Trigger do something similar, albeit on a smaller scale. (In FF5 you're told you need to collect the 12 legendary weapons, but technically you don't need to do that, and only the first tablet (which unlocks 3 of them) is actually mandatory.)
* Romancing SaGa's mandatory tasks consist of a short intro, followed by having to fight a large number of battles to advance time, and then one of the endgame quests will open up. Before then, the game gives you sidequests that you can do so you aren't just wandering around fighting battles to advance time.
* Romancing SaGa 2 and 3, I believe, are still non-linear, but have a bit more structure. RS2, for example, has 7 major bosses that you do have to eventually kill, though not in any particular order.
This pattern also shows up in other games, like Dragon Wars; most of the game is sidequests, but you're expected to do them or there's no way you'll be strong enough to beat the game. (I note that there's one mandatory item and two mandatory spells in Dragon Wars.)