The ability to wear heavy armour and then using Savnok is hardly worth it. Wizards got better ways of defending themselves anyway.
The armor thing isn't the good part... The nice thing about Savnok is just that you wanted Binder for Anima Mage anyway, and you needed heavy armor proficiency to get into the class. The two seriously huge things about Runesmith are the ability to store any unused spells you like for later (and let anyone else in the party cast them when the time comes) and the ability to turn your spells into Spell Like Abilities that will then ignore material components and Xp costs and won't take longer than a Standard Action to cast. That's hugely breakable when you play with it. The more downtime you have, the stronger the Runesmith gets, as he can essentially turn your entire party into casters (spend a few days casting rune spells of personal buffs, then give them to your tank so when a fight starts he can self buff, for example). It's like having the ability to make scrolls entirely for free that don't require UMD to use, with the only restrictions being you can make them two levels lower than your highest casting level and you can only make a few (based on con modifier) per day.
The armor bit is just to make you look like a tank and confuse enemies, but it really doesn't matter much. But hey, free AC is free AC, right?
@KellKheraptis : Never heard of one, but if such an ability exists that would be awesome, both for Runesmiths and for Factotums.
@WarlockLord: Yeah, Anima Mages need to be on that list. Free metamagic and binding is amazing, and if you have access to Zceryll it just goes insane. Note that you can't get every Item Creation feat ever that way, because some have prerequisites... but you can use all the ones you qualify for, and that's still amazing. The best part is that Tainted Sorcerer is only one level long, so you can actually do Binder 1/Wizard 3/Anima Mage 10/Tainted Sorcerer 1/Runesmith 5, getting in the full benefits of three insanely good classes. Free metamagic all day long at no cost, insanely good casting stat, runic casting, free spell likes, and everything else? Yeah, that's completely nuts.
JaronK