Power steering fluid isn't as hygroscopic as brake fluid but it does get contaminated by pump and rack wear. Mercedes puts filters in their power steering systems to filter out those contaminants.
Mercedes has a change interval, and I apply that to every vehicle. I've had several cars go over 200,000 miles on original steering components (pump and rack/box) which I attribute to filtration and fluid changes.
I don't go crazy, but every 50,000-60,000 miles, I do a change.
With a rack for my Volvo's costing over $1,200 (Part only, and friends don't let friends use Cardone rebuilds) and requiring subframe removal to change, I would rather change the fluid periodically and plumb in a filter.