I let them know what they missed by.
In my mind it's not too powerful because they may need a 5 or 6 on the 1d6 and could roll low, their risk, their call.
To each his or her own, but I'm fine with this. The characters would have a vast array of information we (as players) do not have. They'd be able to assess an enemy's combat skill and staying power in a way that players hopefully aren't interested in (other than for the sake of flavor). It's only fair that the players work with the information they have. When I DM live, I have no qualms announcing defenses and approximate HP after a few rounds of combat. Online, I just post that information after the first hit. No sense in hiding the objectives.I do think that the DM needs to learn how the group works.
If they are (like we are in our group) interested in what HP an enemy has to the point that when we add up the damage against him (say 25) and the next hit does 5 points (30 total) that puts him bloodied, we know that he wasnt bloodied at 25 but is at 30 so his total HP are between 50 and 60 and he has between 25 and 30 HP left.
[...]
Also if you hit an opponent for 16 vs Reflex and it misses but another guy hits for 17 vs Reflex and hits, you know that the monster has 17 reflex... it often won't help you since you are going to be at the whim of the dice rolls anyway but sometimes knowing that the same enemy has 22 AC (judging by previous attacks that miss/hit) and 17 reflex, focusing on attacks that hit reflex instead of AC seems wiser.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.