I didn't say you couldn't cast them single handed. I'm just saying that with the longer rod comes more torque on your wrist and arm, I doubt you could cast one all day without being decently fatigued.
And in regards to Spey casting with a switch. Unless you can effectively perform decent to good casts with a Spey rod then you will find the switch harder to figure out. I know with mine it took me a few hours to figure out that I had to change my casting style a bit to make it work well
I have a couple of Echo Switch Rods. The real beauty of a switch and what it was designed for is lifting a good length of line, backcasting and putting it straight out in a single cast. No false casting. With the Airflo 40+ lines I can do this with 70 or 80 feet of line. Shooting 100+ feet of line single or double handed is also very practical.
As for Spey casting. They are better for Skagit casts or Scandi cast as they work a shout 30 foot or so head well. They cast a skagit more than far enough and I have talked to people who chuck 100 feet with a scandi head and poly tips.
The other good thing is they are light and properly balanced feel lighter than a similarly powered single handed rod.