Dreamcatcher said it first, I believe:
It comes down to balance. You expect each PC to throw in 1 - 3 big powers (a couple per-encounter powers, maybe a daily), and then spend the rest of the fight using small powers (at-will powers or even basic attacks, sometimes).
Each encounter you place in your dungeon is scaled in power to expect that level of PC power.
Then, when you run the encounters, you will have some encounters where many PCs roll lots of lucky recharges, and they are able to almost literally spam the encounter and consequently they blow it away easily. Then you'll have some encounters where the opposite happens, and lots of bad rolls causes stuff to not recharge nearly as much as you would expect, and the PCs end up having a very difficult time, maybe even TPK (especially if their bad luck happens in, say, the really challenging climactic fight).
Pbartender said:
So, out of curiosity.
Does it bother anyone that any of the following arguments can be just as valid when applied to DMs and the monsters they pit against player-characters in encounters? If recharging abilities are so troublesome, why have them at all? Why don't monsters have just At-Will powers and Encounter/Daily* powers?
Sure, this kind of good/bad luck that I described can happen to the monsters, too.
But really, if one monster encounter suffers a TPK, well, that was what should have happened anyway, right? Weren't the PCs supposed to win?
It's a whole different story when the PCs suffer a TPK.
In a long, newbie through epic campaign, your PCs will encounter hundreds of encounters. Let's say 300 total encounters. At the end of the campaign, the score should be PCs 300, monsters 0. Sure, maybe you put in 1 or 2 encounters that were designed to whoop the PCs for some reason, so maybe it's more like PCs 298, monsters 2.
But if you make the PCs ability to survive too random, then if even 1% of their battles suffer from horrible luck, that's still 3 TPKs before the campaign ends.
Nobody wants that.