These offer a challenge and break up the monotony. Having to do a hard dungeon with a character I would would never otherwise use requires me, anyway, to use an entirely different lineup and gear choices. The rewards are decent and different. The extra cost because of the bonus though is a bad trade off for the variety this offers.
I’d rather these events had no bonus, a reasonable cost, and ran continuously with no limit on the amount of runs. In my situation I’m running Heartcoil Deeps with level 14 characters and running into level 20 and 21 before the 3rd boss. 2 dice rolls is not worth 525 gold.