Persist is good when either paired with Reach (for static range), or for buffs like Shield (complete immunity to magic missiles and +4 shield bonus to AC). Repeat Spell is good when paired with Solid Fog or Black Tentacles to hold opponents in place, then Repeated Blastomancy to eliminate them with extreme prejeduce.
Best defense? Mirror Image. Beats any Mage Armor with an ugly stick. And this is the single reason why you want Persist as well, because it is normally rounds/level, but with Persist, can last all day long. I'd rather have a flat 12.5% chance of being hit than jacking up my AC against certain attacks. It works against touch attacks (which mage armor doesn't), and is probably worth 40+ AC equivalent worth of odds of opponents missing.