Because you may not be climbing at all at blue line
I do see your point Pace, but the physics don't add up. It sounds a bit like the old "Getting it over the step" myth.
If you aren't climbing at blue line speed, i.e you are only just maintaining altitude, then you are using all the available power. If you don't have excess power to climb, you also don't have excess power to accelerate. If you DO have excess power to accelerate, you are always going to ALSO have excess power to climb.
What you seem to be getting at is keeping some extra margin over blue-line to alow for gusts etc. Fine- but holding a constant speed just above blue-line is going to be more efficient and give you a better net flight path than the "saw tooth" flight path you seem to advocate.
Unfortunatley there are no free lunches in physics- either the power is there or it isn't!