Space Marine sidestepping can be slow, seeing as they wear over two tonnes, if not a tonne, of power armour. Still, I believe it's a good idea.
The armour is called power(ed!) armour for a reason x]. It has internal Gyros, motors etc that enhance the muscular capabilities of the space marine wearing it.
Basically the power armour is very heavy but powers itself in response to the marine's movements and so in effect it is very light to move in.