It really is a safety issue. When it hits the fan, another set of able hands is useful. Apart from anything else a single pilot waiver I believe has to be renewed quite often and there are operational constraints.. .I've had things go wrong, not often happily, in a C550, but I've been happy for the assistance of the chap in the right seat. Ever had a door seal go at F390? Ever had at engine wind down at similar levels? Ever had to review the approach for an alternate when the weather is crap and you would really have liked more fuel?. .Mmmmmm! Two crew rules in my book. Type rated co-pilot preferably and current. Captains of course never become incapacitated do they? Expensive passengers deserve a safety margin.