For a Faction Event, I think there’s already enough incentive to beat as many rooms as you can before defeating the Boss, as each room gives you points. Not beating those rooms means you get fewer points using the same amount of sigils.
Something you might also want to take into account when thinking about this is the relationship between room multipliers and the chance to increase the level of your delve chest. A room that has a higher multiplier than another room of the same difficulty/rarity will also have a lower chance to increase the level of your delve chest - and vice-versa. The main examples that comes to mind here are Goblin Throne Room (x1.2 multiplier, 60% chance) and the one with the Leviathan (I forget, sorry something to do with the Deep/Depths) (x1.5 multiplier, 30%(?) chance). If you hadn’t noticed this, I can understand why you might think some of the higher rarity rooms don’t offer much as compensation for their difficulty.
Are these base amounts that are still multiplied by your cumulative Room Multiplier, or are they independent of it (e.g. on Level 30, each non-boss room will give you 150 points regardless of your multiplier)?