Phil, that's really not true. In most thermals circling is best. Unless the plane has eyes, it doesn't know if it is going upwind or downwind, only that it is moving through the air. If it's moving through the air at a steady speed, as in a circle, it won't rise or sink depending on which direction it's pointed in. Think of sitting in a balloon flying a glider around yourself. The problem is that when you are on the ground, you are moving relative to the air that the glider is in. It's like driving past a merry go round.
In sloping the hill causes the air to be doing different things behind it and in front of it. There are also situations where the air on one side of the circle is doing different things than the other side, as when your circle is not centered on the thermal.
Besides the lack of lift, at many slope soaring sites, particularly ones where the edge of the hill is sharp, the air behind the crest of the hill is all stirred up, at least down low. This is usually called rotor. You can get this just downwind of a line of trees as well.
Check out this gorgeous photo that Paul Naton put on line:
This would be fantastic slope soaring if you had the penthouse suite. However, landings would be tough. You'd have to catch it right at the edge, where there's a whole lot of lift to descend through, or land on the beach. If you tried to land on the roof, you'd get below that nice smooth curve and you'd find the air all stirred up.