For clarity: When I’m talking about discard in this context, I’m talking about effects where the target makes the selection, such as Disrupting Scepter, Raven’s Crime and such.
Ever wondered why discard is generally sorcery-speed? There are some exceptions (such as a couple of charms), but this seems to be the rule. It probably started as a safeguard back in the day, but the rule has remained.
Think about it this way: In most cases, you are better off using it on your turn, when the opponent has less choices. Assuming your opponent has four cards in hand. One of them is the worst card. If you let your opponent draw another card, that might be the worst card and he will discard that. If it’s better than the worst card, you’ve gained nothing. Only your opponent has something to gain here.
Of course, if your opponent has no cards in hand, it would be better to force him to discard during the draw phase, since that would deprive your opponent of his only card for the round, but that would be both unfun and it’s a not a common situation anyway.
I’m not saying discard effects are better at sorcery speed than they would be at instant speed. What I’m saying is that keeping these effects at sorcery speed helps newer players. Seems to me, this is not the only place where Wizards tries to help newer players.
Let’s take exalted, a mechanism most recently seen in M13. It’s not the best mechanism for this, but it encourages attacking. A problem with many less experienced players is that they will often play as defensively as possible. Perhaps they’ve seen a haste-creature once or twice and are willing to exchange doing a few points of damage to possibly evade a few points themselves. But exalted says, maybe you aren’t willing to go all out, but at least this one creature should attack. After all, most decks can’t win without being proactive. Although exalted doesn’t really change that much, it has a psychological effect which might make a difference for those who are not accustomed to thinking ahead and planning ahead to victory.
Of course, at some point, this kind of strong-arming into playing well becomes unnecessary, but I’m willing to bet most players have never thought about why using discard is better during your own turn. On the other hand, once you learn to attack, you’ll never go back (says the aggro-player in me).