If you have just one term, then leaving out the summation sign saves little. But in larger expressions, the rule applies sums per term, and then the savings in clutter can be quite large. For example, here v_i is not summed, and the term with z_b has two sums:

A_ia x_a + v_i + x_a y_a ( w_i + B_ib z_b )

You should not need to check the LHS, x_a y_a should always mean the dot product of these two. At least this is the convention among the heavy addicts; among lighter users you will sometimes find other setups.