Can wizards really cast spells in armor?

Question

I've just read the D&D 5e basic rules, and I'm not sure I'm up to speed regarding casting spells while clad in armor. The only mention I can find about this (page 79):

Because of the mental focus and precise gestures required for spellcasting, you must be proficient with the armor you are wearing to cast a spell. You are otherwise too distracted and physically hampered by your armor for spellcasting.

Does this mean that a character who starts out with a single level fighter (or any other class with heavy armor proficiency) can from then on gain levels as wizard and forever more cast spells, without any penalty whatsoever, while wearing full plate?

I've played all editions of D&D, and this seems both a bit overpowered and very un-D&D-ish to me. Have I missed something?

Answer 1

Yes. Any character who is proficient with a particular type of armor can cast spells while wearing that type of armor with no penalty.

There are two primary reasons for this.

1. There are better ways for mages to get a decent Armor Class. For example, Draconic Sorcerers get permanent Mage Armor for free at 1st level, which provides 13 + Dexterity Modifier AC. That's equivalent AC to a Chain Shirt (except the Chain Shirt will only let you apply a maximum Dexterity bonus of +2), and the Chain Shirt costs 50 gp and weighs 20 pounds. If you take a mage with high Dexterity, then it only gets worse as the armor gets heavier. In addition, any mundane armor heavier than 20 pounds will give you Disadvantage on Stealth checks, and truly heavy armor won't let you apply your Dexterity Modifier at all. Chain, Splint, and Plate Mail all have Strength requirements as well.
2. Arcane Spell Failure was a complicated and pointless mechanic. It only served to confuse newbies and slow down the pace of gameplay. Furthermore, the only spellcasters who have access to metamagic (which, aside from magic gear, was the primary way ASF was mitigated) are Sorcerers, which is also the class that needs heavy armor the least. Magic gear is much harder to come by now, so the likelihood of you finding a magic suit of armor made to be light and maneuverable is much more remote, thus removing yet another way of mitigating ASF.

Answer 2

You understand the rules correctly.

It is neither unbalanced nor a departure from D&D tradition.

It is not unabalanced because there are many ways to increase the AC of a Wizard, from Mage Armor to bracers of defense etc. Armor is expensive, and it requires that the wizard invest in both the armor, and the strength to make use of the armor, as well as the strength or dex and int to multiclass in the first place. They will also suffer from a reduced spell progression table, and will be behind others in the party for stat progression. Ofcourse, you'd be better off taking 1 level of wardomain cleric, but the cost of dipping in regards to stat improvements or feats is still there.

It's not a departure from D&D tradition, because in all versions, if you picked the right combinations you could negate the arcane spell failures for all intents and purposes. The only people who missed out were those who lacked the system mastery to create the character concept. On the other hand, the generic wizard always had a hard time casting spells in various types of armor, and in this edition is is not merely difficult, but impossible to do so unless you choose the correct rule combinations, which allows you to do so. A wizard who is only a wizard, can not cast spells in heavy armor as has always been the case.