Increasing a decreased attribute, Possible? Options

[Seidaku]

Here's a hypothetical situation:

Rat Shaman Roy casts Decrease Strength on Rigger Jim. Jim's strength, normally 5, is reduced to 2 as long as Roy sustains his spell. Bear Shaman Bob wants to help his chummer out, so he casts Increase Strength on Jim to offset Roy's decrease. Since he's rolling against target number 2, he gets 8 successes, raising Jim's strength by 4 (the maximum that his force 4 Increase Strength would allow). During the course of a rather heated combat, Roy takes a burst of gunfire and goes down, releasing his spell in the process. Jim's strength is restored to its original value (5), and Bob's spell (still sustained) is still active. So Jim now has a strength of 9.

Is this valid? If so, why wouldn't you always first decrease an attribute before casting the appropriate increase spell on it? It makes the target numbers much more manageable.