I haven't checked recent clips, but he has lip-synced an awful lot. This suggests that he doesn't think he can sing well enough live. And therefore there's a reason to hire a singer who CAN do it. But I guess in some songs his guitar part isn't so prominent, so it would be weird to be playing a not-so-essential guitar part (Tequila Sunrise, for example) when someone else is singing. Perhaps he wants to have some kind of a lead role in every song since he's a solo performer. I would understand such reasoning.