Yeah, and a charge from a wand that lasts 1 hour is more efficient than one that only lasts 1 minute. You can easily mess up the timing on a 1 minute long shield, and have it drop before it does you any good -- e.g., you detect pesky adventurers approaching, and gleefully prepare; then they stop right at the door, turn around, & wander off. Now your choices are let the spell go to waste, or go chase them down -- and possibly walk into their carefully-prepared-to-kill-you ambush.
Whereas an hour-long mage armor is a nice big window of opportunity; you'll have time to leisurely scope out the situation and decide if they're cunning would-be dragonslayers, or just goofballs with ADD. All without wasting that 15 gp wand charge.
Those hoards don't build themselves, you know. If you go around wasting money, you'll never get that big pile of loot you've always dreamed about.
