Readable Magic always includes magic scrolls found in dungeons. If that's all it's meant to cover, and if magic scrolls can't be used unless Read Magic is cast immediately before the scroll is used, then it's just a "tax" on Magic-Users and doesn't have much utility. An M-U under these conditions gains two benefits:
- Can postpone spell selection until the moment a spell is needed. Example: 2nd level M-U memorizes Read Magic twice and has one scroll for each 1st level spell; the M-U doesn't have to decide in advance whether Sleep will be the most useful, or Charm Person, or Detect Magic, or Protection from Evil.
- Can effectively turn 1st level spell slots into higher-level spell slots. Example: 4th level M-U memorizes Read Magic four times and carries a mix of 1st and 2nd level spell scrolls; instead of being limited to two 2nd level spells on an adventure, the M-U can go as high as six 2nd level spells.
Readable Magic in later editions includes spellbooks. Effectively, it is impossible to decipher a scroll or spellbook unless it's been skimmed using Read Magic. The side effect of this restriction is that, if a Magic-User ever loses all their spellbooks for a given spell level, they can never, ever use spells of that level again. That just seems too restrictive, to me.
A third kind of magical inscription has fallen by the wayside: magical words of command. There are vague hints that some magic items, if not almost all of them, require knowing a magic word. Whether the spellcaster must speak the word every time the item is used or only speak it once to attune the item to their will is not made clear, so it's open to tweaking. Potions would seem to be the only items that definitely don't need a magic word to use, but perhaps magic weapons and armor should also fall into that category. Perhaps only items restricted to M-Us have command words.
I have some more thoughts on this, but I'll reserve those for another post.