Reason 1: blur is only a V component, not an S.
Reason 2: blur is castable on others, and as such provides a useful 2nd level effect spell for cases where you cannot spell the enemy directly (it has strong Sr say) that any fighter would love.
Reason 3: blur is great when fighting rogues. No sneak attacks at all.
Reason 4: the BIGGIE. Magic missiles shred mirror images. Most of the times i have seen mirror image used all it did was draw out a magic missile to drop it. often none of the images lasted a complete round. Fighters would delay their shots waiting for the party sorcerer to drop the MI with MM.
Reason 5: Even if you don't have to worry about 4, blur provides a smaller defense 20% vs 75-80% but over a much longer period. Each Mi spell will block attacks for only a couple rounds, at higher levels, maybe only 1 round as iterative swings cut thru them, while the blur will continue.
All in all, for a sorcerer who needs to qorry about usability and scalability, blur is a much better spell IMO as it doe not get worse with advancement and provides an always useful spell. Your fighters will love you when those wraiths miss their 20% roll.
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All that said...
I typically do not take second lebel spells for defense. Some spell level have good offense. Some spell levels have good defense, some have good utility. Most do not have all three. So i tend to not worry about having "a defensive spell at each level."
My second level spell choices tend to vary on character between alter self, see invis, endurance, cats grace, glitterdust, magic mouth, blindness, invisibility, web, detect thoughts, and yes, even blur. MI was on that list, but no more, not after i saw an empowered MI spell shredded by a MM barrage.
