Silvery barbs doesn't care who casts it. If the fighter takes a critical hit, anyone in the party could cast SB to avert it. They just have to be within 60 feet and able to use a reaction.
So the most logical person to take SB is the one who is most likely to be able to cast it and who has the lowest opportunity cost to a) learn and b) use it. Factors that would go into that decision:
Yes, but playing with a bunch of players this is usually not going to be an options. Also the 60' limit is a pretty big limit.
#1: How much do you have to give up in order to learn and/or prepare SB?
This is the main reason I am probably not getting it as a caster unless it is an AT, and only with an AT because their spell selection is already very limited so SB is really only competing with: Silent Image, Sleep, Disguise Self, Charm Person, Color Spray, Illusory Script and Tasha's hideous Laughter.
The AT gets 2 of these spells at 3rd level and another 1 at 4th. Most of these are pretty mediocre already at 3rd level and trading one of them for SB is not too painful.
With other casters though it is competing against a bunch of good 1st level spells and as you go up in level it becomes an even bigger issue because the cost to keep it known or prepared is a higher level spell. This is what makes the feat such a good option, you aren't giving up anything prepared.
If you look at it this way, I think #1 is a good reason to get it as a Feat or as an arcane trickster, but also a BIG negative to taking it as a spell known/prepared on anything else.
#2: How often can you cast it per day?
Not really an issue if you get it through a feat as any spell you get through a feat will be once a day or once a day plus any slots.
#3: If it costs you a spell slot, how likely are you to want that slot for something else?
This is probably not a big deal. If I am willing to burn a 1st level slot to use it then it is probably worth it and it is a choice I can make when the opportunity presents itself. The preparation cost is a far bigger concern IMO.
#4: How often do you want your reaction for something else?
I disagree with this. To start with it does not take away the ability to use another reaction, it just gives better options.
Noncasters who are not Rogues are probably using their reaction the least in games I play unless they have PAM. The only reaction most of them get is AOOs or ready action when the enemy is still far away. These happen far less than casters using reactions in most of the games I play (including AOOs for those casters) because casters have so many more casting options and many of their control options are timing/situation based and used off turn with ready action.
I would make the opposite argument on #4 - This will allow front liners to use their reaction which would otherwise be wasted and would be better for martials than casters.
#5: How often will you be unconscious?
Probably less if you have it.
#1: You give up just a single spell known or prepared.
But that is huge. Every caster runs out of spells known/prepared. Not every caster runs out of slots.
#3: Once you reach the mid-levels, you can usually spare a 1st-level slot without trouble.
Yes (with a full caster), but by mid level you probably can't spare the spell known or prepared. This is escpecially true if you are a wizard who is taking feats or dexterity ASIs.
This is why a feat is so attractive as an option - no prepared/known slot AND a slot-free casting.
#4: Casters rarely use reactions compared to melee warriors. Now and then you counterspell an enemy caster, or use shield or War Caster if you find yourself in melee, but most of the time your reaction will go unused.
I disagree with this, I think they use them more than melee warriors who are not Rogues, mostly because they have so many more reactions available and are using ready action a lot.
#5: Call this one a wash. The caster is less in harm's way than the front-line warrior, but they also tend to have lower hit points and AC.
I think Wizards generally do not go down as often, in large part because they usually have spells to prevent that. Shield combined with either armor or mage armor is going to usually make AC to be better than most (not all) martials and other spells like Absorb Elements and false life are going to close the hp gap. As a result they generally go down a lot less than others in most games.
This is part of the arguement against giving it to Wizards though. They have other reactions competing and one of them (shield) is going to directly compete any time they are targeted. Using it is a high opportunity cost and could result in the Wizard going down.
Also if you are looking at the "use it for another" arguement there is a high opportunity cost. The fighter gets bit for massive damage on a crit by the Dragon using a legendary action. If the fighter goes down maybe I use it now, if he does not go down though - do I use it now and give up the option of shield later (if the dragon gets to and attacks me), or the option of absorb elements if he breathes on me. I think the opportunity cost is far lower for the fighter, giving up that potential AOO.
#6: This is the one place where a caster is slightly worse than a front-liner; but lots of spells have a range of 60 feet or less, so you're likely to be within that range most of the time.
Probably most of the time, but if you are within 60 feet you are probably going to be getting hit or caught in AOEs a lot too and you have a lot more uses for your reaction.
*In fact, wizard is the single best choice for the silvery barber, partly because "spell prepared" is a lower opportunity cost than "spell known,"
I think it is a higher opportunity cost, especially at higher levels because the Wizzard spells are better. I think it is easier with other clasess because their alternatives are not as good and they are not getting as many new higher level spells per level. If you have a wizard you are generally going to want shield, mage armor and absorb elements. You are also generally going to want feather fall.
You are usually going to want these four your entire career (with the exception of MA if you have armor proficiency).
Add to that, there are a ton of other great 1st level spells you probably want too: PEG, Magic Missile, Cause Fear
Then as you go up in level you really want Rituals in your book so you have them: Identify, Comprehend Languages, Detect Magic ... which means unless you can buy/find scrolls you have a lot of competition just with what you get into your book at 1st and 2nd level.
Even if you get them in, as you go up in level it becomes really hard to keep these all prepared. Every time a wizard gets a level hee gets 2 more spells in his book. Keeping silvery barbs prrepared means 1 less higher level spell prepared every level except 4th and 8th where your ASI evens it. This makes it really difficult to fit SB in there, both in getting it into your book and in preparing it. For example - you make 5th level and you get Fireball and Counterspell but you can only prepare one of them unless you give something else up.
but mainly because the wizard is most likely to have a silvery beard.
