Your Ps2 can stay in the horizontal position with no problems of stability. The system is stable like any other console. So you may ask why did Sony decide to sell this Horizontal stand. In fact this piece of blue plastic has no pratical utilization; it will simply add a more stylish look to the Ps2, lying under the black body of the console.

This Horizontal Stand is identical in its conception (does exist a conception for a blue piece of plastic? yes, it does) to the Vertical Stand, but it's more useless. Anyhow, if you like it, buy it. There is no harm in thinking just in terms of personal tastes.