I have been using Inspiration in my campaign for many things, especially though for awarding players for good roleplay, being creative with what 5e offers, and making sessions fun.
All has worked rather well and I found the rules to be adequate until one of my players used a spell which granted a saving throw. With success would come victory, with failure great sorrow. For me this is one of those occasions where spending Inspiration makes perfect sense as the character (in addition to the player) is throwing in all his weight to succeed.
However, reading the RAW in the PHB page 125:
If you have inspiration, you can expend it when you make an attack roll, saving throw, or ability check. Spending your inspiration gives you advantage on that roll.
When the player announced he wanted to spend his inspiration I came up with the solution of having the enemy roll at disadvantage. This was a spur of the moment thing and I know that as a DM I am well within my right to do this. But if I was to allow it once, I'd want to make it a house rule but not before checking the long-term impact on the game.
Hence I came up with the following reasoning. All of the rolls you get advantage for are made using d20 + Ability Modifier + Proficiency + Misc Bonus if we abstract the miscellaneous bonuses from races and magic items a bit. Seeing as your spell save DC is only 8 + Proficiency + Ability Modifier + Misc Bonus, it seems that you would benefit more from having an enemy roll a save at a disadvantage than roll yourself with advantage. However, I guess it all comes down to what defences your enemy has and I am not keen on drafting an enemy population with a Gaussian distribution regarding their save bonus just to work out the Math.
My question is twofold:
- Did I miss anything in the rules or my considerations which could make my argument above void or alter it significantly?
- Do you think granting players the ability to impose Disadvantage to one enemy (same rolls as they would get Advantage for) rather than having Advantage themselves is a major issue regarding the game mechanics?