I'm a new GM to D&D 4e, and today this situation happened during our game: a player was hit by a Beholder Eye Tyrant, and started taking 2d20 ongoing damage (save ends). The player however changed the damage type of the ongoing damage to be radiant, whilst wearing a customised robe which makes him immune to radiant damage and instead restores half the radiant damage as HP. So far OK, but on his turn he wanted to skip the 'save ends' roll.
I wasn't sure how to handle this. I decided the player had to roll the save ends, and according to What happens if I decide to skip saving? they have no choice but to attempt a save.
I'm wondering, however, if the player should be able to have given up the save ends. If a player's taking 1d6 ongoing fire damage, shouldn't he have the freedom to choose if he wants to put the fire out? Normally that'd be harmful, but in the above example, the character/player knows they'd actually benefit from not saving against the ongoing damage.
So, I understand that according to the rules the saving throw has to happen, but I'm interested to know: What are the repercussions of allowing players to skip saving throws for the Save Ends on effects? What are the repercussions for players, the GM, etc?