Potential bonuses for Shields?

A 1% bonus is only worth about 2-2.5 weapon attack or magic attack (after BB). It's not a huge advantage. A 3 line shield is significantly better, though.

A 3 line is definitely better, but getting 2 lines as main stat is defenitely more common than 3 lines main stat, so it's better and it costs less.

Edit: that's true, Sapta. Still, for thieves and magicians it holds true.