I read somewhere that Mithril's effect on armor lets certain casters where higher grade armor and not suffer arcane failure. So if proficient a Summoner could were a Mithril breast plate and still cast. Rather than burn the valuable feat I was wondering if a 1 level dip into either Fighter or some sort of Cavalier would be worth it? I know you lose out on some things but for Unchained Summoner would it really make that much of a difference?
First off, it's martial, not marshal.
Armour has an Arcane Spell Failure percent chance on it. Look up the specific piece of armour, and then look up the Mithral special material to see how much it reduces ASF percentage (it lowers it by 10%).
What exactly do you mean by "front line" Summoner, anyway?
As I understand it, your question basically boils down to this:
• As a summoner, you can cast in light armor.
• A mithral breastplate is "one category lighter than normal for purposes of movement and other limitations," which arguably includes the ability to cast in it if you can cast in light armor.
• But you're not proficient in medium armor, so you would suffer the armor check penalty (-1 in this case) on attack rolls and Strength- and Dexterity-based ability checks as well as on skill checks.
• So is it worth it to dip fighter, say, to get proficiency to mitigate this penalty?
If I understand the question correctly, the answer is almost certainly no. There are a couple of traits which can mitigate that last point of armor check penalty for you if you must. If you don't want to spend the trait, either, you can (eventually) afford the comfort enhancement to solve the problem instead.
Note: I suppose you should check with your DM to see if you can cast in a mithral breastplate if you can cast in light armor. I would've said obviously so, but what do I know?
Mithral is a rare, silvery metal that is lighter than steel but just as hard. When worked like steel, it can be used to create amazing armor, and is occasionally used for other items as well. Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor.
At no time dose it change the type of armor it is. Summoners can cast in light armor with out fail. Breastplate is medium armor. You suffer the 15% spell fail.
One would argue "other limitations" includes armored casting.
For those half-casters who can cast unhindered in light armor specifically (summoner, bard, magus,. ..), a mithral breastplate should do the trick. Like it would for a rogues's evasion and other similar things.
You'd still need medium proficiency though, since that is indeed the one thing not helped by mithral (or some other way to deal with non-proficiency and its side effects).
Grab the feat, or go for trait cheese. The benefit is not enough to sacrifice spell/eidolon progression.
As noted, expect table variation. At my table, a summoner could cast in a mithral breastplate.
Here's my reasoning. There are several limitations depending on armor category:
• Without some ability that says otherwise, a character's movement in medium or heavy armor is slowed.
• Without some ability that says otherwise, a character who sleeps in medium or heavy armor is fatigued the next day.
• If you're not proficient in the armor you're wearing, you apply the armor check penalty not only to Strength- and Dexterity-based skill checks, but also to attack rolls and Strength- and Dexterity-based ability checks. Note: an initiative check is a Dexterity check, so I would apply your armor check penalty to your initiative roll if wearing an armor in which you're not proficient.
• Several class abilities depend on the type of armor you're wearing (e.g. a rogue's evasion, as Nyerkh pointed out, or ranger's combat style, or a summoner's ability to cast without suffering arcane spell failure).
We have to decide how each of these rules elements interact with those for making armor out of mithral.
One approach is to say that mithral medium armor is still actually medium armor for all purposes except movement, so you can't comfortably sleep in it, a rogue can't use evasion in it, and so forth. I believe this reading to be wrong, because it gives no effect to the phrase "and other limitations."
Another approach is to treat mithral medium armor as light armor for purposes of movement and some other cases, and as medium armor for purposes of proficency and some other cases. I don't think this approach is tenable either, because it is essentially asking the DM to make ad hoc judgments with no rules guidance at all.
So my approach is to say that mithral medium armor counts as light armor for all purposes except those for which the rules explicitly say it doesn't, i.e. for purposes of proficiency. I believe this interpretation to be the most compelling because it gives effect to every phrase in the rules regarding mithral armor and requires no ad hoc decisions on my part.
But YMMV, and so, as always: ask your DM.