Scrolls have been redesigned to hold rituals. But in earlier editions, they used to let a caster cast a spell without expending a memorized spell. That's the likely reason it was changed in 4E - it messes with the expected amount of special abilities a character can bring into combat.
A normal 4E character brings a pair of at-will powers, some encounter powers, a few daily powers, some utility powers that may be useful, and whatever powers are attached to his items (and you can only use a few item powers per day). Adding scrolls to that loadout might double the amount of special powers, or more. Clearly, combat scrolls need to be limited in some way if you care about balance.
So my way to implement spell scrolls would be like this:
- A scroll lets you use a specific power once, then the scroll is spent.
- You have to be able to use a power of the scroll power's level, and be using the proper power source. (This means Wizards and Warlocks can use the same "Arcane" scrolls, and Clerics and Paladins can use "Divine" scrolls.)
- To cast from the scroll, you expend one of your powers of the same level or higher, with the same regain rate (at-will, encounter or daily) and with the same power source (matters for multiclassers and half-elves).
- Other than expending one of your power uses, the spell will work as if you had had the power and cast it normally. It uses the same action types, keywords, etc.
(Using the proper power source is optional. Fighting Men (and Women) whipping out a scroll and unleashing arcane hell is cool too.)
For example, let's look at a scroll of Fireball, a level 5 Daily from the Wizard list.
Scroll of Fireball
This scroll is warm to the touch, and the writing glows in the dark.
Price: 50 gp
Power (Consumable): Free Action. Read this scroll. You expend one unspent Daily Arcane power of at least level 5 and cast Fireball (as per the Player's Handbook) instead of that power.
(The description of Fireball would go here, if it weren't likely to be copyrighted.)
There we go. A wizard with this scroll doesn't get more spells in one combat, but he gets to switch out a less useful spell for Fireball. And a Warlock is even happier since he normally can't know Fireball.